It was returning 0 when it should be returning an empty string. The 'AssertEqual' in the ale image is from an old version so it does not check the types of the arguments. This is already fixed in https://github.com/junegunn/vader.vim/commit/427fe19104c15066e4c1d5d385076e8e07a0dee8 Closes #3120
https://scalameta.org/metals/