I get an error with an incorrect error message when I try to create a signature with a notation data secion with an empty value.
Reading section 5.2.3.16 of rfc 4880 i can't find anything that says that it shouldn't be allowed.
Example of failure, first succeeds second fails with an error:
capitol@tool:~$ echo 1 | gpg -se --sig-notation \!test@hackeriet.no=message -r 0x1D108E6C07CBC406 > /tmp/test.gpg capitol@tool:~$ echo 1 | gpg -se --sig-notation \!test@hackeriet.no= -r 0x1D108E6C07CBC406 > /tmp/test2.gpg gpg: WARNING: unable to %-expand notation (too large). Using unexpanded.