diff --git a/pkg/tanka/evaluators.go b/pkg/tanka/evaluators.go
index e8ed91980..5aa1443b5 100644
--- a/pkg/tanka/evaluators.go
+++ b/pkg/tanka/evaluators.go
@@ -20,6 +20,11 @@ func evalJsonnet(path string, impl types.JsonnetImplementation, opts jsonnet.Opt
 
 	// evaluate Jsonnet
 	if opts.EvalScript != "" {
+		// Determine if the entrypoint is a function.
+		isFunction, err := jsonnet.Evaluate(path, impl, fmt.Sprintf("std.isFunction(import '%s')", entrypoint), opts)
+		if err != nil {
+			return "", fmt.Errorf("evaluating jsonnet in path '%s': %w", path, err)
+		}
 		var tla []string
 		for k := range opts.TLACode {
 			tla = append(tla, k+"="+k)
@@ -29,7 +34,7 @@ func evalJsonnet(path string, impl types.JsonnetImplementation, opts jsonnet.Opt
   %s
 `, entrypoint, opts.EvalScript)
 
-		if len(tla) != 0 {
+		if isFunction == "true\n" {
 			tlaJoin := strings.Join(tla, ", ")
 			evalScript = fmt.Sprintf(`
 function(%s)
diff --git a/pkg/tanka/evaluators_test.go b/pkg/tanka/evaluators_test.go
index f6ce72417..0802a2152 100644
--- a/pkg/tanka/evaluators_test.go
+++ b/pkg/tanka/evaluators_test.go
@@ -52,3 +52,61 @@ func TestEvalJsonnetWithExpression(t *testing.T) {
 		})
 	}
 }
+
+// An EvalScript with a top-level function containing only optional arguments
+// should be evaluated as a function even if no TLAs are provided.
+func TestEvalWithOptionalTlas(t *testing.T) {
+	opts := jsonnet.Opts{
+		EvalScript: "main.metadata.name",
+	}
+	json, err := evalJsonnet("testdata/cases/with-optional-tlas/main.jsonnet", jsonnetImpl, opts)
+	assert.NoError(t, err)
+	assert.Equal(t, `"bar-baz"`, strings.TrimSpace(json))
+}
+
+// An EvalScript with a top-level function containing should allow passing only
+// a subset of the TLAs.
+func TestEvalWithOptionalTlasSpecifiedArg2(t *testing.T) {
+	opts := jsonnet.Opts{
+		EvalScript: "main.metadata.name",
+		TLACode:    jsonnet.InjectedCode{"baz": "'changed'"},
+	}
+	json, err := evalJsonnet("testdata/cases/with-optional-tlas/main.jsonnet", jsonnetImpl, opts)
+	assert.NoError(t, err)
+	assert.Equal(t, `"bar-changed"`, strings.TrimSpace(json))
+}
+
+// An EvalScript with a top-level function having no arguments should be
+// evaluated as a function even if no TLAs are provided.
+func TestEvalFunctionWithNoTlas(t *testing.T) {
+	opts := jsonnet.Opts{
+		EvalScript: "main.metadata.name",
+	}
+	json, err := evalJsonnet("testdata/cases/function-with-zero-params/main.jsonnet", jsonnetImpl, opts)
+	assert.NoError(t, err)
+	assert.Equal(t, `"inline"`, strings.TrimSpace(json))
+}
+
+// An EvalScript with a top-level function should return an understandable
+// error message if an incorrect TLA is provided.
+func TestInvalidTlaArg(t *testing.T) {
+	opts := jsonnet.Opts{
+		EvalScript: "main",
+		TLACode:    jsonnet.InjectedCode{"foo": "'bar'"},
+	}
+	json, err := evalJsonnet("testdata/cases/function-with-zero-params/main.jsonnet", jsonnetImpl, opts)
+	assert.Contains(t, err.Error(), "function has no parameter foo")
+	assert.Equal(t, "", json)
+}
+
+// Providing a TLA to an EvalScript with a non-function top level mainfile
+// should not return an error.
+func TestTlaWithNonFunction(t *testing.T) {
+	opts := jsonnet.Opts{
+		EvalScript: "main",
+		TLACode:    jsonnet.InjectedCode{"foo": "'bar'"},
+	}
+	json, err := evalJsonnet("testdata/cases/withenv/main.jsonnet", jsonnetImpl, opts)
+	assert.NoError(t, err)
+	assert.NotEmpty(t, json)
+}
diff --git a/pkg/tanka/testdata/cases/function-with-zero-params/main.jsonnet b/pkg/tanka/testdata/cases/function-with-zero-params/main.jsonnet
new file mode 100644
index 000000000..6227cd3d2
--- /dev/null
+++ b/pkg/tanka/testdata/cases/function-with-zero-params/main.jsonnet
@@ -0,0 +1,16 @@
+function() {
+  apiVersion: 'tanka.dev/v1alpha1',
+  kind: 'Environment',
+  metadata: {
+    name: 'inline',
+  },
+  spec: {
+    apiServer: 'https://localhost',
+    namespace: 'inline',
+  },
+  data: {
+    apiVersion: 'v1',
+    kind: 'ConfigMap',
+    metadata: { name: 'config' },
+  },
+}
diff --git a/pkg/tanka/testdata/cases/with-optional-tlas/main.jsonnet b/pkg/tanka/testdata/cases/with-optional-tlas/main.jsonnet
new file mode 100644
index 000000000..5294d75da
--- /dev/null
+++ b/pkg/tanka/testdata/cases/with-optional-tlas/main.jsonnet
@@ -0,0 +1,16 @@
+function(bar='bar', baz='baz') {
+  apiVersion: 'tanka.dev/v1alpha1',
+  kind: 'Environment',
+  metadata: {
+    name: bar + '-' + baz,
+  },
+  spec: {
+    apiServer: 'https://localhost',
+    namespace: 'inline',
+  },
+  data: {
+    apiVersion: 'v1',
+    kind: 'ConfigMap',
+    metadata: { name: 'config' },
+  },
+}