Trying to populate field on a form based on selection from another combobox

I am trying to make a field on a form named txtWORKING_INTEREST fill with a DMax from a table.  The code I'm trying is:

Me.txtWORKING_INTEREST = DMax("[WORKING_INTEREST]", "tblPropertyInterests", "[PROPNUM] = '" & Forms!frmRecordsHeader!txtPROPNUM & "'")

And the table, tblPropertyInterests" looks like this for txtPROPNUM:


But the after update event keeps filling the field txtWORKING_INTEREST  with 9.350474  (which is a text field as is txtPROPNUM on the form).

What have I missed?