Tabbing Through Integer Field Causes Error

I have a form with several fields that are updated to a DB as an integer. I am using this code to validate the fields:

iincidentdet = CInt(iincidentdet)
If Err.number <> 0 Then
strErrorMsg = strErrorMsg & "Your entry for integer_field could " & _
"not be converted to an integer! Remember that integers " & _
"need to be from -32,768 to 32,767.<br />" & vbCrLf
Err.Clear
End If

when I use my tab to pass by one of the integer fields which should be a 'zero' or null. Is there a way to default the fields to a null value?