Unable to replace Null values using Python parser of ArcGIS 10.0 Field Calculator?

I'm using the following python code to replace Null values by 0, using field calculator:

def nulls(p1):
 if p1 is None:
    return 0
 else:
    return p1

But nothing happens, Null values stay. I'm using ArcGIS Desktop 10.0.

Answer

Since your code worked for me, a work around would be to use 'select by attribute' to select null rows e.g. MyField IS NULL. Once you have them all selected just use field calculator e.g. MyField = 0.