diff --git a/lambda/invoke_loop.go b/lambda/invoke_loop.go
index 25fc6fa4..3f679ba5 100644
--- a/lambda/invoke_loop.go
+++ b/lambda/invoke_loop.go
@@ -13,9 +13,8 @@ import (
 )
 
 const (
-	serializationErrorFormat = `{"errorType": "Runtime.SerializationError", "errorMessage": "%s"}`
-	msPerS                   = int64(time.Second / time.Millisecond)
-	nsPerMS                  = int64(time.Millisecond / time.Nanosecond)
+	msPerS  = int64(time.Second / time.Millisecond)
+	nsPerMS = int64(time.Millisecond / time.Nanosecond)
 )
 
 // startRuntimeAPILoop will return an error if handling a particular invoke resulted in a non-recoverable error
@@ -48,7 +47,7 @@ func handleInvoke(invoke *invoke, function *Function) error {
 	}
 
 	if functionResponse.Error != nil {
-		payload := safeMarshal(functionResponse.Error)
+		payload := mustMarshal(functionResponse.Error)
 		if err := invoke.failure(payload, contentTypeJSON); err != nil {
 			return fmt.Errorf("unexpected error occured when sending the function error to the API: %v", err)
 		}
@@ -100,10 +99,13 @@ func convertInvokeRequest(invoke *invoke) (*messages.InvokeRequest, error) {
 	return res, nil
 }
 
-func safeMarshal(v interface{}) []byte {
+// mustMarshal is like json.Marshal but panics if the v cannot be marshaled.
+// use this if you know the v doesn't contain values that json.Marshal can't marshal,
+// such as Channel, complex, and function values.
+func mustMarshal(v interface{}) []byte {
 	payload, err := json.Marshal(v)
 	if err != nil {
-		return []byte(fmt.Sprintf(serializationErrorFormat, err.Error()))
+		panic("lambda: " + err.Error())
 	}
 	return payload
 }