How to fix “WARNING: not a detached signature”?

I am trying to sign a Python file using the following:

gpg --sign --output ./ulagen.py.sig ./ulagen.py 

When I verify the signature using gpg --verify ./ulagen.py.sig, I get the following error:

gpg: WARNING: not a detached signature; file 'ulagen.py' was NOT verified! 

What am I doing wrong?