git add after changing a submodule

If I make a change to a submodule and commit it, I then have to add it
to the parent repo.  If I do

git add path/to/submodule

then it works fine.  But if I do

git add path/to/submodule/ (with trailing slash)

then it blows away the submodule and adds all the files in that directory.

Seems pretty dangerous to me.  I know that I can just use git commit
-a to do it, but I'd like to prevent the mistake of adding the path
and screwing up the repo.  Is it possible to protect the submodules a
bit more?

Pat