change on form with VBA produces errorcode Run-time error '94': Invalid use of Null

[kelkan]

I am trying to have a form update certain fields based on the user changing a field value. Below is the code that I have written.

Private Sub CBOCOMPLETE_AfterUpdate()
If IsNull(CLASSDATE) Then
CLASSDATE = InputBox("You must enter the date this class was completed.")
End If
If CLASSCOMPLETED = "YES" Then
Me.CLASSHOURS = Me.COURSEHOURS
ElseIf CLASSCOMPLETED <> "YES" Then
Me.CLASSHOURS = 0
End If
Dim strdate As String
strdate = DateAdd("M", Me.FREQUENCY, Me.CLASSDATE)


If Me.COURSECATEGORY = "PRIMARY CERTIFICATION" And Me.CLASSCOMPLETED = "YES" Then
Me.EXPIRATIONDATE = strdate
ElseIf COURSECATEGORY = "SECONDARY CERTIFICATION" And Me.CLASSCOMPLETED = "YES" Then
Me.EXPIRATIONDATE = strdate
ElseIf Me.COURSECATEGORY <> "PRIMARY CERTIFICATION" Or "SECONDARY CERTIFICATION" And CLASSCOMPLETED = "YES" Then
Me.CLASSHOURS = Me.COURSEHOURS And Nz(EXPIRATIONDATE)

End If

On Error Resume Next
End Sub

This works fine if the class has an expiration date. Unfortunately, many of my classes do not have or require an expiration date and that's when I get the error message. Ideas on how to correct?

[kelkan]

Disregard - I made some minor changes and fixed it myself...

Private Sub CBOCOMPLETE_AfterUpdate()
If IsNull(CLASSDATE) Then
CLASSDATE = InputBox("You must enter the date this class was completed.")
End If

If Me.CLASSCOMPLETED = "YES" Then
Me.CLASSHOURS = Me.COURSEHOURS
ElseIf Me.CLASSCOMPLETED <> "YES" Then
Me.CLASSHOURS = 0
End If


Dim strdate As String
strdate = DateAdd("M", Me.FREQUENCY, Me.CLASSDATE)
If Me.COURSECATEGORY = "PRIMARY CERTIFICATION" And Me.CLASSCOMPLETED = "YES" Then
Me.EXPIRATIONDATE = strdate
End If

If Me.COURSECATEGORY = "SECONDARY CERTIFICATION" And Me.CLASSCOMPLETED = "YES" Then
Me.EXPIRATIONDATE = strdate
End If

If Me.COURSECATEGORY = "MANDATORY" Or Me.COURSECATEGORY = "OPTIONAL" And Me.CLASSCOMPLETED = "YES" Then
Nz (EXPIRATIONDATE)
End If

On Error Resume Next
End Sub