gitk: searching on filenames?

A real example from this morning:  I noticed when pulling from
Linus that kernel/sched.c had been updated, so I wanted to look
at that particular commit.

If I do 'gitk kernel/sched.c' then it does what I expect.

But, if I just start gitk with no arguments and then attempt
to search for kernel/sched.c it eventually stops with an error
saying "can't find diffs for <SHA1> <SHA2>".

Is this the expected behavior?  (I have the 'Files' field
selected in the gitk search bar.)

Thanks!