diff --git a/act/exprparser/functions.go b/act/exprparser/functions.go
index 64103e07..37a38db5 100644
--- a/act/exprparser/functions.go
+++ b/act/exprparser/functions.go
@@ -151,7 +151,7 @@ func (impl *interperterImpl) join(array reflect.Value, sep reflect.Value) (strin
 
 func (impl *interperterImpl) toJSON(value reflect.Value) (string, error) {
 	if value.Kind() == reflect.Invalid {
-		return "", nil
+		return "null", nil
 	}
 
 	json, err := json.MarshalIndent(value.Interface(), "", "  ")
diff --git a/act/exprparser/functions_test.go b/act/exprparser/functions_test.go
index bdae533e..3c3d3477 100644
--- a/act/exprparser/functions_test.go
+++ b/act/exprparser/functions_test.go
@@ -137,6 +137,7 @@ func TestFunctionToJSON(t *testing.T) {
 		name     string
 	}{
 		{"toJSON(env) }}", "{\n  \"key\": \"value\"\n}", "toJSON"},
+		{"toJSON(null)", "null", "toJSON-null"},
 	}
 
 	env := &EvaluationEnvironment{