calling git rebase with invalid onto-ref exits with wrong error message

Just noticed a small bug (in git version 1.7.10.msysgit.1):

calling
   git rebase --onto <onto-ref> <base-ref> <branch-ref>

with an unknown <onto-ref> reports:
   fatal: Needed a single revision
   Does not point to a valid commit: <branch-ref>

Expected:
   fatal: Needed a single revision
   Does not point to a valid commit: <onto-ref>

Cheers,
Manuela