My Travis keeps erroring

Question

All tests are passing, but in in the end, it gives

The command "make html" failed and exited with 2 during .

This happens since I started using cache. Initially I just had

cache:
  apt: true
  directories:
    - $HOME/virtualenv/python2.7/lib/python2.7/site-packages
    - node_modules

in travis.yml

but I also added - $HOME/virtualenv/bin and cleared my cache in master branch before I ran another build. It passed once, but then keeps failing. I cleared the cache multiple times but no avail.