Can't checkout master branch

Can't checkout master branch

I've been working in a branch with some code to improve an application.
Now that I this is finished, I want to merge this branch with master
branch, but I can't. I first try to checkout master branch like this:
$ git checkout master
but I get this error:
error: The following untracked working tree files would be overwritten by
checkout:
vendor/bin/doctrine
vendor/bin/doctrine.php
...
but the vendor directory is included in .gitignore file. If I do:
$ git status
I get this:
# On branch mejoras_contralador
nothing to commit (working directory clean)
After this, I would do:
$ git merge branch
How to avoid this error?