androidTestCompile mockito givning Error

I'm trying to use Mockito for instrumentation tests, when I add

androidTestCompile 'org.mockito:mockito-core:2.7.22'

I get the following error:

Error:Conflict with dependency 'org.objenesis:objenesis' in project ':app'. 
Resolved versions for app (2.1) and test app (2.5) differ. See 
http://ift.tt/1ZPcybz for details.

I have no clue what's wrong, I have no org.objenesis dependency specified anywhere in my code (guess it comes through mockito).

Every help, appreciated.