diff --git a/handler.go b/handler.go
index 61930e1..446fb18 100644
--- a/handler.go
+++ b/handler.go
@@ -20,18 +20,21 @@ var (
 func init() {
 	var handleRequestCallback js.Func
 	handleRequestCallback = js.FuncOf(func(this js.Value, args []js.Value) any {
-		if len(args) > 1 {
-			panic(fmt.Errorf("too many args given to handleRequest: %d", len(args)))
-		}
 		reqObj := args[0]
 		var cb js.Func
 		cb = js.FuncOf(func(_ js.Value, pArgs []js.Value) any {
 			defer cb.Release()
 			resolve := pArgs[0]
+			reject := pArgs[1]
 			go func() {
+				if len(args) > 1 {
+					reject.Invoke(jsutil.Errorf("too many args given to handleRequest: %d", len(args)))
+					return
+				}
 				res, err := handleRequest(reqObj)
 				if err != nil {
-					panic(err)
+					reject.Invoke(jsutil.Error(err.Error()))
+					return
 				}
 				resolve.Invoke(res)
 			}()
@@ -58,7 +61,7 @@ func handleRequest(reqObj js.Value) (js.Value, error) {
 	}
 	req, err := jshttp.ToRequest(reqObj)
 	if err != nil {
-		panic(err)
+		return js.Value{}, err
 	}
 	ctx := runtimecontext.New(context.Background(), reqObj)
 	req = req.WithContext(ctx)
@@ -90,7 +93,5 @@ func Serve(handler http.Handler) {
 	}
 	httpHandler = handler
 	ready()
-	select {
-	case <-closeCh:
-	}
+	<-closeCh
 }
diff --git a/internal/jsutil/jsutil.go b/internal/jsutil/jsutil.go
index 1175029..e93a0af 100644
--- a/internal/jsutil/jsutil.go
+++ b/internal/jsutil/jsutil.go
@@ -34,6 +34,14 @@ func NewPromise(fn js.Func) js.Value {
 	return PromiseClass.New(fn)
 }
 
+func Error(msg string) js.Value {
+	return ErrorClass.New(msg)
+}
+
+func Errorf(format string, args ...any) js.Value {
+	return ErrorClass.New(fmt.Sprintf(format, args...))
+}
+
 // ArrayFrom calls Array.from to given argument and returns result Array.
 func ArrayFrom(v js.Value) js.Value {
 	return ArrayClass.Call("from", v)
diff --git a/internal/jsutil/stream.go b/internal/jsutil/stream.go
index ed4b4b0..79f5e51 100644
--- a/internal/jsutil/stream.go
+++ b/internal/jsutil/stream.go
@@ -134,8 +134,7 @@ func (rs *readerToReadableStream) Pull(controller js.Value) error {
 		return nil
 	}
 	if err != nil {
-		jsErr := ErrorClass.New(err.Error())
-		controller.Call("error", jsErr)
+		controller.Call("error", Error(err.Error()))
 		if err := rs.reader.Close(); err != nil {
 			return err
 		}
@@ -170,7 +169,7 @@ func ConvertReaderToReadableStream(reader io.ReadCloser) js.Value {
 			go func() {
 				err := stream.Pull(controller)
 				if err != nil {
-					reject.Invoke(ErrorClass.New(err.Error()))
+					reject.Invoke(Error(err.Error()))
 					return
 				}
 				resolve.Invoke()
@@ -180,11 +179,22 @@ func ConvertReaderToReadableStream(reader io.ReadCloser) js.Value {
 		return NewPromise(cb)
 	}))
 	rsInit.Set("cancel", js.FuncOf(func(js.Value, []js.Value) any {
-		err := stream.Cancel()
-		if err != nil {
-			panic(err)
-		}
-		return js.Undefined()
+		var cb js.Func
+		cb = js.FuncOf(func(this js.Value, pArgs []js.Value) any {
+			defer cb.Release()
+			resolve := pArgs[0]
+			reject := pArgs[1]
+			go func() {
+				err := stream.Cancel()
+				if err != nil {
+					reject.Invoke(Error(err.Error()))
+					return
+				}
+				resolve.Invoke()
+			}()
+			return js.Undefined()
+		})
+		return NewPromise(cb)
 	}))
 	return ReadableStreamClass.New(rsInit)
 }