Expression has no value error

Hello, I have a form named frmcalendar with a subform named 
frmReviewReminderSubForm and I want the subform to be visible= false if a 
control named txtDateOfHire on my subform is null. The control is a date
so 
im getting an error "You entered an expression that has no value" here is
the 
on open event im using. Thanks!

If Me.frmReviewReminderSubForm.Form!txtDateOfHire = "" Then
  Me.frmReviewReminderSubForm.Visible = True
  Else
  Me.frmReviewReminderSubForm.Visible = False
  End If