strange test for None

[karoly.kiripolszky]

in my server i use the following piece of code:

            ims = self.headers["if-modified-since"]
            if ims != None:
                t = int(ims)

and i'm always getting the following error:

    t = int(ims)
ValueError: invalid literal for int(): None

i wanna know what the hell is going on... first i tried to test using
is not None, but it makes no difference.

sorry i forgot, it's interpreter 2.4.4.