Gitattributes not properly supported with add command

The following:

git add -u :\(glob,attr:-someAttr\):src/**

Produces an error that, according to the source code, should never be visible to the user. This attribute/pathspec *should* be supported according to the documentation provided by git:

fatal: BUG:builtin/add.c:498: unsupported magic 40

It looks like the documentation claims more features than the source code supports, perhaps incorrectly because I think you shouldn't be restricted to a set of attributes, and the source code 
doesn't properly do its job anyways and never handles this scenario.

This should be fixed, if I have any time I'll look into what it would take to submit a patch, but I don't have the time for this right now.