Bug in function CalcOverdueBalance in T:Customer OR????

Hi to all,

I add new fields in T: 454 Approval Entry :50000 Balance (LCY) Decimal and 50001 Overdue Balance (LCY) Decimal.

In the codeunit 439: Approval Managment in function MakeApprovalEntry in the end I add this code:

IF TableID = DATABASE::"Sales Header" THEN
BEGIN
SalesHeader.GET(DocType, DocNo);
SalesReceivablesSetup.GET;
Cust.GET(SalesHeader."Bill-to Customer No.");
Cust.CALCFIELDS("Balance (LCY)");
"Balance (LCY)" := Cust."Balance (LCY)";
"Overdue Balance (LCY)" := Cust.CalcOverdueBalance;
MODIFY;
END;

Now the customer say that for some customers works good, but for some customers the "Overdue Balance (LCY)"=0 (aslo I see and I can't understand how that happen, the customer have a "Overdue Balance (LCY)" but there is zero ).
I use NAV 2015

Any advice,
Thanks a lot