def RemoveNULL(x):
    if x is None:
        return ''
    elif x == '':
        return '0'
    else: return x

RemoveNULL(str( !DEAD_VOL_PER_HA_SPP1_125!))

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

def replaceNull(x):
  if x is None:
    return 0
  else:
    return x

replaceNull(!YourFieldName!)