#Error With Function In Query

Hi All,

I made a function to calculate the difference in hours between two times.

Function TimeDiff(Time1 As String, Time2 As String) As Double
'This function is used to calculate the difference in hours between two times.
'It is capable of handling a finish time that is after midnight.

TimeDiff = 0

If Not IsNull(Time1) And Not IsNull(Time2) Then

If CDate(Time1) > CDate(Time2) Then
TimeDiff = ((CDate(Time2) - CDate(Time1)) + 1) * 24
Else
TimeDiff = (CDate(Time2) - CDate(Time1)) * 24
End If

End If

End Function
I have a query where there are 2 sets of times in fields (Start1, Finish1, Start2, Finish2).
When I add a calculated column with the following expression:
Expr1: TimeDiff(Start1,Finish1) + TimeDiff(Start2,Finish2)
it calculates in the query correctly.

However, when I made a new function that incorporated this function, it gave me the #error result in the calculated field.
Function OrdinaryHours(Status As String, Start1 As String, Finish1 As String, Optional Start2 As String, Optional Finish2 As String) As Double

'Initialise
OrdinaryHours = 0

'If employee is NOT a casual
If Not Status = "CT" Then

If Not IsMissing(Start2) And Not IsMissing(Finish2) Then
OrdinaryHours = TimeDiff(Start1, Finish1) + TimeDiff(Start2, Finish2)
Else
OrdinaryHours = TimeDiff(Start1, Finish1)
End If

End If


End Function
Does anyone know why this would #Error. I have the TimeDiff function initialising to = 0 before doing anything...I am stumped why it returns #Error when it should return 0 in the case of something going wrong.

Brad