Replacing NULL value with Zero in geodatabase table using Python parser of ArcGIS field calculator?

I am having some difficulties to change "NULL" values to zero in geodatabase. I have tried the following python code in ArcGIS 10.1 ("Field Calculator") to change it but did not work.

Pre-Logic Script Code:

def RemoveNULL(x):

    if x is None:
        return ''
    elif x == '':
        return '0'
    else: return x

Calculation field:

RemoveNULL(str( !DEAD_VOL_PER_HA_SPP1_125!))

Error message:

ERROR: 999999: Error executing function
The value type is incompatible with the field type. [dead_pine]
Failed to execute (calculate field)