Tests fail ONLY on one python version, and the diffs only show README change. How's that possible?

I am developing a python package. Every commit is tested on Travis CI on python 3.5 and 3.6. At one point, the tests started failing on 3.5 only. Diffs only show minor README changes. I rerun the build a couple of times with no change. The error logs show a discrepancy between the signatures of two functions. Since I am dynamically building these objects, I know the order of arguments depend on the stability of the sorted library function. This is the only hunch I have. Any ideas? Thanks!