diff --git a/pkg/sinks/forward/forward.go b/pkg/sinks/forward/forward.go
index 400b076734..ecb0f7c133 100644
--- a/pkg/sinks/forward/forward.go
+++ b/pkg/sinks/forward/forward.go
@@ -351,18 +351,18 @@ func (df *DataForward) writeToSink(ctx context.Context, sinkWriter sinker.SinkWr
 			if err = errs[idx]; err != nil {
 				// if we are asked to write to fallback sink, check if the fallback sink is configured,
 				// and we are not already in the fallback sink write path.
-				if errors.As(err, &udsink.WriteToFallbackErr) && df.opts.fbSinkWriter != nil && !isFbSinkWriter {
+				if errors.Is(err, &udsink.WriteToFallbackErr) && df.opts.fbSinkWriter != nil && !isFbSinkWriter {
 					fallbackMessages = append(fallbackMessages, msg)
 					continue
 				}
 
 				// if we are asked to write to fallback but no fallback sink is configured, we will retry the messages to the same sink
-				if errors.As(err, &udsink.WriteToFallbackErr) && df.opts.fbSinkWriter == nil {
+				if errors.Is(err, &udsink.WriteToFallbackErr) && df.opts.fbSinkWriter == nil {
 					df.opts.logger.Error("Asked to write to fallback but no fallback sink is configured, retrying the message to the same sink")
 				}
 
 				// if we are asked to write to fallback sink inside the fallback sink, we will retry the messages to the fallback sink
-				if errors.As(err, &udsink.WriteToFallbackErr) && isFbSinkWriter {
+				if errors.Is(err, &udsink.WriteToFallbackErr) && isFbSinkWriter {
 					df.opts.logger.Error("Asked to write to fallback sink inside the fallback sink, retrying the message to fallback sink")
 				}
 
diff --git a/pkg/sinks/udsink/errors.go b/pkg/sinks/udsink/errors.go
index b6844279b2..3caddd0607 100644
--- a/pkg/sinks/udsink/errors.go
+++ b/pkg/sinks/udsink/errors.go
@@ -32,15 +32,15 @@ type InternalErr struct {
 }
 
 // IsUserUDSinkErr is true if the problem is due to the user code in the UDSink.
-func (e ApplyUDSinkErr) IsUserUDSinkErr() bool {
+func (e *ApplyUDSinkErr) IsUserUDSinkErr() bool {
 	return e.UserUDSinkErr
 }
 
 // IsInternalErr is true if this is a platform issue. This is a blocking error.
-func (e ApplyUDSinkErr) IsInternalErr() bool {
+func (e *ApplyUDSinkErr) IsInternalErr() bool {
 	return e.InternalErr.Flag
 }
 
-func (e ApplyUDSinkErr) Error() string {
+func (e *ApplyUDSinkErr) Error() string {
 	return fmt.Sprint(e.Message)
 }
diff --git a/pkg/sinks/udsink/udsink_grpc.go b/pkg/sinks/udsink/udsink_grpc.go
index 9b1a61627e..d52105823e 100644
--- a/pkg/sinks/udsink/udsink_grpc.go
+++ b/pkg/sinks/udsink/udsink_grpc.go
@@ -94,7 +94,7 @@ func (u *UDSgRPCBasedUDSink) ApplySink(ctx context.Context, requests []*sinkpb.S
 	response, err := u.client.SinkFn(ctx, requests)
 	if err != nil {
 		for i := range requests {
-			errs[i] = ApplyUDSinkErr{
+			errs[i] = &ApplyUDSinkErr{
 				UserUDSinkErr: false,
 				Message:       fmt.Sprintf("gRPC client.SinkFn failed, %s", err),
 				InternalErr: InternalErr{
@@ -112,19 +112,19 @@ func (u *UDSgRPCBasedUDSink) ApplySink(ctx context.Context, requests []*sinkpb.S
 	}
 	for i, m := range requests {
 		if r, existing := resMap[m.GetId()]; !existing {
-			errs[i] = NotFoundErr
+			errs[i] = &NotFoundErr
 		} else {
 			if r.GetStatus() == sinkpb.Status_FAILURE {
 				if r.GetErrMsg() != "" {
-					errs[i] = ApplyUDSinkErr{
+					errs[i] = &ApplyUDSinkErr{
 						UserUDSinkErr: true,
 						Message:       r.GetErrMsg(),
 					}
 				} else {
-					errs[i] = UnknownUDSinkErr
+					errs[i] = &UnknownUDSinkErr
 				}
 			} else if r.GetStatus() == sinkpb.Status_FALLBACK {
-				errs[i] = WriteToFallbackErr
+				errs[i] = &WriteToFallbackErr
 			} else {
 				errs[i] = nil
 			}
diff --git a/pkg/sinks/udsink/udsink_grpc_test.go b/pkg/sinks/udsink/udsink_grpc_test.go
index 0a7fbe60e7..b367416cc5 100644
--- a/pkg/sinks/udsink/udsink_grpc_test.go
+++ b/pkg/sinks/udsink/udsink_grpc_test.go
@@ -114,7 +114,7 @@ func Test_gRPCBasedUDSink_ApplyWithMockClient(t *testing.T) {
 		assert.Equal(t, 2, len(gotErrList))
 		assert.Equal(t, nil, gotErrList[0])
 
-		assert.Equal(t, ApplyUDSinkErr{
+		assert.Equal(t, &ApplyUDSinkErr{
 			UserUDSinkErr: true,
 			Message:       "mock sink message error",
 		}, gotErrList[1])
@@ -158,7 +158,7 @@ func Test_gRPCBasedUDSink_ApplyWithMockClient(t *testing.T) {
 		u := NewMockUDSgRPCBasedUDSink(mockClient)
 		gotErrList := u.ApplySink(ctx, testDatumList)
 		expectedErrList := []error{
-			ApplyUDSinkErr{
+			&ApplyUDSinkErr{
 				UserUDSinkErr: false,
 				Message:       "gRPC client.SinkFn failed, failed to execute c.grpcClt.SinkFn(): mock SinkFn error",
 				InternalErr: InternalErr{
@@ -166,7 +166,7 @@ func Test_gRPCBasedUDSink_ApplyWithMockClient(t *testing.T) {
 					MainCarDown: false,
 				},
 			},
-			ApplyUDSinkErr{
+			&ApplyUDSinkErr{
 				UserUDSinkErr: false,
 				Message:       "gRPC client.SinkFn failed, failed to execute c.grpcClt.SinkFn(): mock SinkFn error",
 				InternalErr: InternalErr{
diff --git a/pkg/sources/forward/data_forward_test.go b/pkg/sources/forward/data_forward_test.go
index 0dbf09cdf4..4f69728e14 100644
--- a/pkg/sources/forward/data_forward_test.go
+++ b/pkg/sources/forward/data_forward_test.go
@@ -1161,7 +1161,7 @@ func (f myForwardInternalErrTest) WhereTo(_ []string, _ []string, s string) ([]f
 }
 
 func (f myForwardInternalErrTest) ApplyTransform(_ context.Context, _ *isb.ReadMessage) ([]*isb.WriteMessage, error) {
-	return nil, udfapplier.ApplyUDFErr{
+	return nil, &udfapplier.ApplyUDFErr{
 		UserUDFErr: false,
 		InternalErr: struct {
 			Flag        bool
diff --git a/pkg/sources/transformer/grpc_transformer.go b/pkg/sources/transformer/grpc_transformer.go
index 9b4b437d3c..fe15a75c49 100644
--- a/pkg/sources/transformer/grpc_transformer.go
+++ b/pkg/sources/transformer/grpc_transformer.go
@@ -113,7 +113,7 @@ func (u *GRPCBasedTransformer) ApplyTransform(ctx context.Context, readMessage *
 				return true, nil
 			})
 			if !success {
-				return nil, rpc.ApplyUDFErr{
+				return nil, &rpc.ApplyUDFErr{
 					UserUDFErr: false,
 					Message:    fmt.Sprintf("gRPC client.SourceTransformFn failed, %s", err),
 					InternalErr: rpc.InternalErr{
@@ -123,7 +123,7 @@ func (u *GRPCBasedTransformer) ApplyTransform(ctx context.Context, readMessage *
 				}
 			}
 		case sdkerr.NonRetryable:
-			return nil, rpc.ApplyUDFErr{
+			return nil, &rpc.ApplyUDFErr{
 				UserUDFErr: false,
 				Message:    fmt.Sprintf("gRPC client.SourceTransformFn failed, %s", err),
 				InternalErr: rpc.InternalErr{
@@ -132,7 +132,7 @@ func (u *GRPCBasedTransformer) ApplyTransform(ctx context.Context, readMessage *
 				},
 			}
 		default:
-			return nil, rpc.ApplyUDFErr{
+			return nil, &rpc.ApplyUDFErr{
 				UserUDFErr: false,
 				Message:    fmt.Sprintf("gRPC client.SourceTransformFn failed, %s", err),
 				InternalErr: rpc.InternalErr{
diff --git a/pkg/sources/transformer/grpc_transformer_test.go b/pkg/sources/transformer/grpc_transformer_test.go
index 66259e246e..c74a5a88ea 100644
--- a/pkg/sources/transformer/grpc_transformer_test.go
+++ b/pkg/sources/transformer/grpc_transformer_test.go
@@ -173,7 +173,7 @@ func TestGRPCBasedTransformer_BasicApplyWithMockClient(t *testing.T) {
 			ReadOffset: isb.SimpleStringOffset(func() string { return "0" }),
 		},
 		)
-		assert.ErrorIs(t, err, rpc.ApplyUDFErr{
+		assert.ErrorIs(t, err, &rpc.ApplyUDFErr{
 			UserUDFErr: false,
 			Message:    fmt.Sprintf("%s", err),
 			InternalErr: rpc.InternalErr{
@@ -227,7 +227,7 @@ func TestGRPCBasedTransformer_BasicApplyWithMockClient(t *testing.T) {
 			ReadOffset: isb.SimpleStringOffset(func() string { return "0" }),
 		},
 		)
-		assert.ErrorIs(t, err, rpc.ApplyUDFErr{
+		assert.ErrorIs(t, err, &rpc.ApplyUDFErr{
 			UserUDFErr: false,
 			Message:    fmt.Sprintf("%s", err),
 			InternalErr: rpc.InternalErr{
@@ -277,7 +277,7 @@ func TestGRPCBasedTransformer_BasicApplyWithMockClient(t *testing.T) {
 			ReadOffset: isb.SimpleStringOffset(func() string { return "0" }),
 		},
 		)
-		assert.ErrorIs(t, err, rpc.ApplyUDFErr{
+		assert.ErrorIs(t, err, &rpc.ApplyUDFErr{
 			UserUDFErr: false,
 			Message:    fmt.Sprintf("%s", err),
 			InternalErr: rpc.InternalErr{
@@ -377,7 +377,7 @@ func TestGRPCBasedTransformer_BasicApplyWithMockClient(t *testing.T) {
 			ReadOffset: isb.SimpleStringOffset(func() string { return "0" }),
 		},
 		)
-		assert.ErrorIs(t, err, rpc.ApplyUDFErr{
+		assert.ErrorIs(t, err, &rpc.ApplyUDFErr{
 			UserUDFErr: false,
 			Message:    fmt.Sprintf("%s", err),
 			InternalErr: rpc.InternalErr{
diff --git a/pkg/udf/forward/forward_test.go b/pkg/udf/forward/forward_test.go
index 3a0b2febbe..65c57ee05f 100644
--- a/pkg/udf/forward/forward_test.go
+++ b/pkg/udf/forward/forward_test.go
@@ -1601,7 +1601,7 @@ func (f myForwardInternalErrTest) WhereTo(_ []string, _ []string, s string) ([]f
 }
 
 func (f myForwardInternalErrTest) ApplyMap(_ context.Context, _ *isb.ReadMessage) ([]*isb.WriteMessage, error) {
-	return nil, udfapplier.ApplyUDFErr{
+	return nil, &udfapplier.ApplyUDFErr{
 		UserUDFErr: false,
 		InternalErr: struct {
 			Flag        bool
@@ -1613,7 +1613,7 @@ func (f myForwardInternalErrTest) ApplyMap(_ context.Context, _ *isb.ReadMessage
 
 func (f myForwardInternalErrTest) ApplyMapStream(_ context.Context, _ *isb.ReadMessage, writeMessagesCh chan<- isb.WriteMessage) error {
 	close(writeMessagesCh)
-	return udfapplier.ApplyUDFErr{
+	return &udfapplier.ApplyUDFErr{
 		UserUDFErr: false,
 		InternalErr: struct {
 			Flag        bool
diff --git a/pkg/udf/rpc/errors.go b/pkg/udf/rpc/errors.go
index c711dcd09d..cfdffe5757 100644
--- a/pkg/udf/rpc/errors.go
+++ b/pkg/udf/rpc/errors.go
@@ -32,20 +32,20 @@ type InternalErr struct {
 }
 
 // IsUserUDFErr is true if the problem is due to the user code in the UDF.
-func (e ApplyUDFErr) IsUserUDFErr() bool {
+func (e *ApplyUDFErr) IsUserUDFErr() bool {
 	return e.UserUDFErr
 }
 
 // IsInternalErr is true if this is a platform issue. This is a blocking error.
-func (e ApplyUDFErr) IsInternalErr() bool {
+func (e *ApplyUDFErr) IsInternalErr() bool {
 	return e.InternalErr.Flag
 }
 
-func (e ApplyUDFErr) Error() string {
+func (e *ApplyUDFErr) Error() string {
 	return fmt.Sprint(e.Message)
 }
 
 // Is checks if the error is of the same type
-func (e ApplyUDFErr) Is(target error) bool {
+func (e *ApplyUDFErr) Is(target error) bool {
 	return target.Error() == e.Error()
 }
diff --git a/pkg/udf/rpc/grpc_aligned_reduce.go b/pkg/udf/rpc/grpc_aligned_reduce.go
index 51d0ccc023..1a774306eb 100644
--- a/pkg/udf/rpc/grpc_aligned_reduce.go
+++ b/pkg/udf/rpc/grpc_aligned_reduce.go
@@ -204,14 +204,14 @@ func createAlignedReduceRequest(windowRequest *window.TimedWindowRequest) *reduc
 }
 
 // convertToUdfError converts the error returned by the reduceFn to ApplyUDFErr
-func convertToUdfError(err error) ApplyUDFErr {
+func convertToUdfError(err error) error {
 	// if any error happens in reduce
 	// will exit and restart the numa container
 	udfErr, _ := sdkerr.FromError(err)
 	switch udfErr.ErrorKind() {
 	case sdkerr.Retryable:
 		// TODO: currently we don't handle retryable errors for reduce
-		return ApplyUDFErr{
+		return &ApplyUDFErr{
 			UserUDFErr: false,
 			Message:    fmt.Sprintf("gRPC client.ReduceFn failed, %s", err),
 			InternalErr: InternalErr{
@@ -220,7 +220,7 @@ func convertToUdfError(err error) ApplyUDFErr {
 			},
 		}
 	case sdkerr.NonRetryable:
-		return ApplyUDFErr{
+		return &ApplyUDFErr{
 			UserUDFErr: false,
 			Message:    fmt.Sprintf("gRPC client.ReduceFn failed, %s", err),
 			InternalErr: InternalErr{
@@ -229,7 +229,7 @@ func convertToUdfError(err error) ApplyUDFErr {
 			},
 		}
 	case sdkerr.Canceled:
-		return ApplyUDFErr{
+		return &ApplyUDFErr{
 			UserUDFErr: false,
 			Message:    context.Canceled.Error(),
 			InternalErr: InternalErr{
@@ -238,7 +238,7 @@ func convertToUdfError(err error) ApplyUDFErr {
 			},
 		}
 	default:
-		return ApplyUDFErr{
+		return &ApplyUDFErr{
 			UserUDFErr: false,
 			Message:    fmt.Sprintf("gRPC client.ReduceFn failed, %s", err),
 			InternalErr: InternalErr{
diff --git a/pkg/udf/rpc/grpc_aligned_reduce_test.go b/pkg/udf/rpc/grpc_aligned_reduce_test.go
index 1288c21bd1..1cdd00bf5c 100644
--- a/pkg/udf/rpc/grpc_aligned_reduce_test.go
+++ b/pkg/udf/rpc/grpc_aligned_reduce_test.go
@@ -199,7 +199,7 @@ func TestGRPCBasedUDF_AsyncReduceWithMockClient(t *testing.T) {
 					break readLoop
 				}
 				if err != nil {
-					assert.ErrorIs(t, err, ApplyUDFErr{
+					assert.ErrorIs(t, err, &ApplyUDFErr{
 						UserUDFErr: false,
 						Message:    err.Error(),
 						InternalErr: InternalErr{
diff --git a/pkg/udf/rpc/grpc_map.go b/pkg/udf/rpc/grpc_map.go
index 01d049c64a..5b6251907d 100644
--- a/pkg/udf/rpc/grpc_map.go
+++ b/pkg/udf/rpc/grpc_map.go
@@ -110,7 +110,7 @@ func (u *GRPCBasedMap) ApplyMap(ctx context.Context, readMessage *isb.ReadMessag
 				return true, nil
 			})
 			if !success {
-				return nil, ApplyUDFErr{
+				return nil, &ApplyUDFErr{
 					UserUDFErr: false,
 					Message:    fmt.Sprintf("gRPC client.MapFn failed, %s", err),
 					InternalErr: InternalErr{
@@ -120,7 +120,7 @@ func (u *GRPCBasedMap) ApplyMap(ctx context.Context, readMessage *isb.ReadMessag
 				}
 			}
 		case sdkerr.NonRetryable:
-			return nil, ApplyUDFErr{
+			return nil, &ApplyUDFErr{
 				UserUDFErr: false,
 				Message:    fmt.Sprintf("gRPC client.MapFn failed, %s", err),
 				InternalErr: InternalErr{
@@ -129,7 +129,7 @@ func (u *GRPCBasedMap) ApplyMap(ctx context.Context, readMessage *isb.ReadMessag
 				},
 			}
 		default:
-			return nil, ApplyUDFErr{
+			return nil, &ApplyUDFErr{
 				UserUDFErr: false,
 				Message:    fmt.Sprintf("gRPC client.MapFn failed, %s", err),
 				InternalErr: InternalErr{
diff --git a/pkg/udf/rpc/grpc_map_test.go b/pkg/udf/rpc/grpc_map_test.go
index 05fd5d5c2d..ea8c4b36af 100644
--- a/pkg/udf/rpc/grpc_map_test.go
+++ b/pkg/udf/rpc/grpc_map_test.go
@@ -183,7 +183,7 @@ func TestGRPCBasedMap_BasicApplyWithMockClient(t *testing.T) {
 			ReadOffset: isb.SimpleStringOffset(func() string { return "0" }),
 		},
 		)
-		assert.ErrorIs(t, err, ApplyUDFErr{
+		assert.ErrorIs(t, err, &ApplyUDFErr{
 			UserUDFErr: false,
 			Message:    fmt.Sprintf("%s", err),
 			InternalErr: InternalErr{
@@ -233,7 +233,7 @@ func TestGRPCBasedMap_BasicApplyWithMockClient(t *testing.T) {
 			ReadOffset: isb.SimpleStringOffset(func() string { return "0" }),
 		},
 		)
-		assert.ErrorIs(t, err, ApplyUDFErr{
+		assert.ErrorIs(t, err, &ApplyUDFErr{
 			UserUDFErr: false,
 			Message:    fmt.Sprintf("%s", err),
 			InternalErr: InternalErr{
@@ -337,7 +337,7 @@ func TestGRPCBasedMap_BasicApplyWithMockClient(t *testing.T) {
 			ReadOffset: isb.SimpleStringOffset(func() string { return "0" }),
 		},
 		)
-		assert.ErrorIs(t, err, ApplyUDFErr{
+		assert.ErrorIs(t, err, &ApplyUDFErr{
 			UserUDFErr: false,
 			Message:    fmt.Sprintf("%s", err),
 			InternalErr: InternalErr{
diff --git a/pkg/udf/rpc/grpc_mapstream.go b/pkg/udf/rpc/grpc_mapstream.go
index e6287fb6fa..360422df17 100644
--- a/pkg/udf/rpc/grpc_mapstream.go
+++ b/pkg/udf/rpc/grpc_mapstream.go
@@ -89,7 +89,7 @@ func (u *GRPCBasedMapStream) ApplyMapStream(ctx context.Context, message *isb.Re
 	errs.Go(func() error {
 		err := u.client.MapStreamFn(ctx, d, responseCh)
 		if err != nil {
-			err = ApplyUDFErr{
+			err = &ApplyUDFErr{
 				UserUDFErr: false,
 				Message:    fmt.Sprintf("gRPC client.MapStreamFn failed, %s", err),
 				InternalErr: InternalErr{
diff --git a/pkg/udf/rpc/grpc_mapstream_test.go b/pkg/udf/rpc/grpc_mapstream_test.go
index 186cdfdd96..6329ec149f 100644
--- a/pkg/udf/rpc/grpc_mapstream_test.go
+++ b/pkg/udf/rpc/grpc_mapstream_test.go
@@ -176,7 +176,7 @@ func TestGRPCBasedUDF_BasicApplyStreamWithMockClient(t *testing.T) {
 			},
 			ReadOffset: isb.SimpleStringOffset(func() string { return "0" }),
 		}, writeMessageCh)
-		assert.ErrorIs(t, err, ApplyUDFErr{
+		assert.ErrorIs(t, err, &ApplyUDFErr{
 			UserUDFErr: false,
 			Message:    fmt.Sprintf("%s", err),
 			InternalErr: InternalErr{
diff --git a/pkg/udf/rpc/grpc_unaligned_reduce_test.go b/pkg/udf/rpc/grpc_unaligned_reduce_test.go
index 947e3aaf7e..1b9c7dd1e2 100644
--- a/pkg/udf/rpc/grpc_unaligned_reduce_test.go
+++ b/pkg/udf/rpc/grpc_unaligned_reduce_test.go
@@ -172,7 +172,7 @@ func TestGRPCBasedUDF_BasicSessionReduceWithMockClient(t *testing.T) {
 					break readLoop
 				}
 				if err != nil {
-					assert.ErrorIs(t, err, ApplyUDFErr{
+					assert.ErrorIs(t, err, &ApplyUDFErr{
 						UserUDFErr: false,
 						Message:    err.Error(),
 						InternalErr: InternalErr{
diff --git a/test/e2e/functional_test.go b/test/e2e/functional_test.go
index e70fe9a3ea..11078a7f36 100644
--- a/test/e2e/functional_test.go
+++ b/test/e2e/functional_test.go
@@ -1,5 +1,3 @@
-//go:build test
-
 /*
 Copyright 2022 The Numaproj Authors.
 
@@ -342,6 +340,23 @@ func isWatermarkProgressing(ctx context.Context, client *daemonclient.DaemonClie
 	return true, nil
 }
 
+func (s *FunctionalSuite) TestFallbackSink() {
+
+	w := s.Given().Pipeline("@testdata/simple-fallback.yaml").
+		When().
+		CreatePipelineAndWait()
+	defer w.DeletePipelineAndWait()
+	pipelineName := "simple-fallback"
+
+	// send a message to the pipeline
+	w.SendMessageTo(pipelineName, "in", NewHttpPostRequest().WithBody([]byte("fallback-message")))
+
+	// wait for all the pods to come up
+	w.Expect().VertexPodsRunning()
+
+	w.Expect().SinkContains("output", "fallback-message")
+}
+
 func TestFunctionalSuite(t *testing.T) {
 	suite.Run(t, new(FunctionalSuite))
 }
diff --git a/test/e2e/testdata/simple-fallback.yaml b/test/e2e/testdata/simple-fallback.yaml
new file mode 100644
index 0000000000..372fc4d29c
--- /dev/null
+++ b/test/e2e/testdata/simple-fallback.yaml
@@ -0,0 +1,32 @@
+apiVersion: numaflow.numaproj.io/v1alpha1
+kind: Pipeline
+metadata:
+  name: simple-fallback
+spec:
+  vertices:
+    - name: in
+      source:
+        http: {}
+    - name: cat
+      scale:
+        min: 1
+      udf:
+        builtin:
+          name: cat
+    - name: output
+      scale:
+        min: 1
+      sink:
+        udsink:
+          container:
+            image: quay.io/numaio/numaflow-go/fb-sink-log:stable
+            imagePullPolicy: Always
+        fallback:
+          udsink:
+            container:
+              image: quay.io/numaio/numaflow-sink/redis-e2e-test-sink:latest
+  edges:
+    - from: in
+      to: cat
+    - from: cat
+      to: output