Error Transformer for GraphQL query/mutation doesn't work as expected?

I am a newbie here. What I am trying to do is
1\ Use Error Transformer (on a GraphQL query/mutation) to return the error message
2\ Get the error message in Javascript query, which calls the GraphQL query/mutation, and display the error message in a text widget.

From console output, I can see that the Error Transform does retrieve the error message; however, what the Javascript query gets is way more than the error message, such as metadata ... For example,

{"data":{"data":{"xxx":{"addUser":{"id":"yyy","message":"zzz"},"user":null}},"metadata": ....}

Shouldn't I get the message "zzz" only ? Anything I misunderstand or miss about Error Transformer? Please HELP!

The Error Transformer is as follows (xxx is GraphQL query/mutation name)

if (Array.isArray(errors) && errors.length > 0) {
  return errors[0].message;
} else if (data.addUser.error) {
  return data.addUser.error.message;
} else {
  return false;

The Javascript query that calls GraphQL query/mutation is as follows

return addUser.trigger({
  additionalScope: {input: input},
  onSuccess: (data) => {
  onFailure: (error) => {

Hey, @happyFish could you please write in to our support chat so we can take a closer look at what exactly may be going on here? As soon as we have it figured out we will update this post. Thank You!