[].include?(:test) return nil [RoR]

Question

Having the strange problem that [].include?(:test) returns nil instead of expected false. The whole thing only happens when running the app (i see it in rubymine debug mode), not in irb or rails console.

I tested following

[].include?(:test) # nil

[].include?(:test).nil? # nil

[].class # Array

Seems as if include? is overwritten somewhere, but I have not been able to find out where. grep -R Array * and grep -R include? * show nothing strange. Any idea how I could find out more?

Thanks