Formula not reading Duration correctly?

I'm trying to use "Flag1" to show "Yes" for rows to show and "No" for rows hide. "Yes" goes for the rows with a Duration bigger than 0 or that has a Deadline set. I'm using this formula:

IIf(([Duration]>0) Or (isdate([Deadline])=True);"Yes";"No")

Why do I get "No" for many rows (not all though!) that has a zero Duration and no Deadline?

Using Project 2010.

Thanks!