Forms :: Conditionally Using Asterisk In A Query Formed From Strings?

I have a button on a Form which opens a report, passing in two WHERE clauses which are driven by two comboboxes on the Form (one for ProductType and one for ProductCollection).

Everything works great but I needed to add an "All" option to the comboboxes so that the user can get all Producttypes from a specific Collection or vice versa.

I've managed to add the "All" option to one of the comboboxes (using the UNION query method on the rowsource). I'm a bit stumped though, as to how I would actually tweak the function below to do a select * where the combobox value is found to be = All.

Code:
Private Sub ProduceReport_Click()
Dim strWhere As String
strWhere = "producttype = " & Me.ProductTypeForReport
strWhere = strWhere & " AND ProductCollectionName = '" & Me.CollectionForReport
strWhere = strWhere & "'"
Debug.Print strWhere
DoCmd.OpenReport "Products Report", acViewReport, , strWhere
End Sub