I got the same error in my Android app when I tried adding a second set of calls that touched my version of provideRetrofit(). The original caller needed a ScalarsConverterFactory and the new caller needed a GsonConverterFactory. I solved it by checking the request URL for a specific keyword that only appears in the scalar request to flip the converter factory accordingly. Not a stellar solution but hopefully this helps someone else if they have the same issue with the converter factories.