The posted expression seems to invoke too many control(s) and forms references. I've had trouble before with the bracketed syntax for referring to subform controls, so I don't use it. To drill down to a subform control, I always use this syntax
[Forms]![Main form name]![subform control name].[Form]![control name on subform] where [subform control name] is the subform control and NOT the subform. Hopefully, you don't allow these to have the same name.
To reference a property of same (e.g. record count), I found this always worked (whereas the former didn't for some reason)
Forms("MainFormName").Controls("subformControlName ").Form.Recordset.Recordcount
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.