Forms :: Form Unload Error

I'm getting an error on some fairly simple code attached to a form's Unload event, but I'm only seeing it when I switch my form to design mode. My code does look like it's working under normal conditions,I've attached a simplified database that reproduces the problem in all its glory. I've searched for similar problems and solutions but none have quite hit the spot.

The code is supposed to prevent users closing a form if they haven't entered at least one record in the subform. In my attached example, you shouldn't be able to close the form if you are currently on a Person record that has no linked Hobby record. Do do this, I have used the following code in the Unload event of frmPerson:

Code:

Private Sub Form_Unload(Cancel As Integer)
If Me.frmhobby.Form.Recordset.RecordCount < 1 Then
Call MsgBox("you must enter at least one hobby for this person")
Cancel = True
End If
End Sub

To see it working properly, go to Person record 3 (Laura Blackwood) which has no Hobby records, and try to close the form. You can't close it until you have entered a hobby for this person. As far as I can see the code works as expected.

Now navigate to Person Record 4 (Mary McArthur). Instead of trying to close the form, switch the form to Design View. You will get the following error: "Runtime error 2455: You entered an expression that has an invalid reference to the property form/report". Return to Form view and navigate to record 2 (Kevin Petersen), then switch back to Design View - same error. Interestingly, you don't ever get the error on Record 1. All this is exactly what happens on my real database too.

So I'm worried that my code isn't right despite appearances. I thought switching to Design View just goes through the same Unload-Deactivate-Close routine as when you close a form in a conventional way? I also know that subforms unload after forms, so since the code is attached to the Form, the subform should still be available to it and shouldn't result in a reference error.