This should be fairly simple, but never for me.  All I want to do is have my
MsgBox macro to pop up and say "No data for this report".  My macro works
fine, but I cannot figure out the code for where I need it to pop up.  

The basis is a simple form with text dates beginning and ending, plus two
drop down boxes with WorkCenter# OR ClassCode# (just basic 4 digit numbers
each).  Then there is the option to print a report based on the criteria I
select.  One report button for ClassCode# and a second button for
WorkCenter#.  If the operator fails to put in a ClassCode# (for instance),
then selects the ClassCode# report button, the macro message box should pop
up with the above statement.  How and where would I insert that macro request
into the code of the form?  I tried this, but no luck:
Private Sub Report_by_WorkCenter#_Click()
On Error GoTo Err_Report_by_WorkCenter#_Click

   Dim stDocName As String
   
   If IsNull(Me!cbxWorkCenter#) Then
   DoCmd mcrMsgBox
   Exit Sub
   End If

   stDocName = "rptMFG QC Scrap Detail by WorkCenter#"
   DoCmd.OpenReport stDocName, acPreview

Exit_Report_by_WorkCenter#_Click:
   Exit Sub

Err_Report_by_WorkCenter#_Click:
   MsgBox Err.Description
   Resume Exit_Report_by_WorkCenter#_Click
   
End Sub

Any suggestions is greatly appreciated.