diff --git a/tailbone/views/vendors/invoices.py b/tailbone/views/vendors/invoices.py
index 8293c2c9..2bd9e624 100644
--- a/tailbone/views/vendors/invoices.py
+++ b/tailbone/views/vendors/invoices.py
@@ -110,7 +110,10 @@ class VendorInvoiceCrud(FileBatchCrud):
         Let the invoice handler in effect determine if the user-provided
         purchase order number is valid.
         """
-        parser = require_invoice_parser(field.parent.parser_key.value)
+        parser_key = field.parent.parser_key.value
+        if not parser_key:
+            raise formalchemy.ValidationError("Cannot validate PO number until File Type is chosen")
+        parser = require_invoice_parser(parser_key)
         vendor = get_vendor(Session(), parser.vendor_key)
         try:
             self.handler.validate_po_number(value, vendor)