Code runs correctly, yet fails

I would like to give some feedback about ...

From: https://checkio.org/mission/the-longest-palindromic/solve/

HTTP_USER_AGENT:

Mozilla/5.0 (Macintosh; Intel Mac OS X 10_6_8) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/49.0.2623.112 Safari/537.36

My code for the longest palindromic challenge runs correctly, and will print the correct return value but it is failing the test for some reason.