Vendor Shipment No. already exist for this vendor ..

Hello,

I get this error "Vendor Shipment No. already exist  for this vendor" after i try to post purchase Invoice .I ran debugger .it breaks at  below mentioned. i codeunit 90 Purch.-Post. on run trigger.I checked the table  PurchRcptHeader. there is no value in Vendor Shipment No. error text  148 is the same that's cmng.any one can suggest the reason..thanks.

      IF Receive THEN BEGIN

        PurchRcptHeader.RESET;

        PurchRcptHeader.SETRANGE("Buy-from Vendor No.","Buy-from Vendor No.");

        PurchRcptHeader.SETRANGE("Vendor Shipment No.","Vendor Shipment No.");

        PurchRcptHeader.SETRANGE("Gate Entry Block",FALSE);

        IF PurchRcptHeader.FIND('-') THEN

         ERROR(Text148,PurchRcptHeader."Vendor Shipment No.");

      END;