Conditional formating between two forms 1

[DevelopV]

I have a form "frmCustomerPaymentFindInvoiceOrder" with an unbound subform "frmCustomerPaymentFindInvoice"
There are no linked master / child fields

In the main form is a combo box "cboCustomer" that shows a list of customers. The first column of the combo box is the CustomerId
The data source for the subform also contains the CustomerId and the criteria for this field is [Forms]![frmCustomerPaymentFindInvoiceOrder]![cboCustomer]

In the main form is an unbound control called "PaymentValue"

In the sub form is a control called "InvoiceValue"

I wish to apply conditional formatting to the control "InvoiceValue", so if "InvoiceValue" = "PaymentValue" then the background colour of "InvoiceValue" is red

I have opened "frmCustomerPaymentFindInvoice" in Layout View, and selected Conditional formatting. Selected field "InvoiceValue" and set the condition to "Field Value Is Equal to Forms![frmCustomerPaymentFindInvoiceOrder]![PaymentValue].
I have also selected a background colour.

However the conditional formatting does not work!

Any suggestions on how to fix this?

 

[MajP]

Expression is"
Forms!frmCustomerPaymentFindInvoiceOrder.cboCustomer = [invoiceValue]