Problem: If you ever encounter "Message was not processed: Undefined(-1)" error when you look into the audit log as shown in the below screen shot.
Root Cause: One of the root cause of this error is to check the response header size/length (print the same in the audit log and see the total number of characters in the header) and if it exceeds 8012 character then you hit the gateway default header size (as per testing on API Gateway 9.2). If you try reducing the header size to either 8012 or less, then see if you can get the response, if yes, then it is definitely sure that this header is the cause of this issue.
Here's the quick google search on the header size limit with most web app servers.
Solution: Increase the header size limit by setting the property maxHttpHeaderSize=<new value> in Listen Port Properties under Advanced Tab as shown in the below screen shot and then retry executing the service, it should work without any issue.