Calculat the averge cost

I created a function that calculates the average cost of an article this report works as follows:
1: I set "Startdate" and "enddate"
2: I select the item I want to have its average cost during this period
but the problem is that my function looks Document No. as purchase during select it finds no purchase well gives me a Run-Time Error 0 / 0

Here is my source code:


GetPMP(VAR ItemNo : Code[25];VAR StartDate : Date;VAR EndDate : Date) : Decimal

ItemLedgerEntry.SETRANGE(ItemLedgerEntry."Posting Date",StartDate,EndDate) ;
ItemLedgerEntry.SETFILTER(ItemLedgerEntry."Item No.",'%1',ItemNo);
ItemLedgerEntry.SETFILTER(ItemLedgerEntry."Entry Type",'%1',ItemLedgerEntry."Entry Type"::Purchase);
ItemLedgerEntry.SETFILTER(ItemLedgerEntry."Document No.",'%1','AR*');
IF ItemLedgerEntry.FINDFIRST THEN
SumQuantity:=0 ;
SumSales:=0;
REPEAT
ItemLedgerEntry.CALCFIELDS(ItemLedgerEntry."Cost Amount (Actual)");
BEGIN
SumQuantity:=SumQuantity+ItemLedgerEntry.Quantity;
MESSAGE(FORMAT(SumQuantity));
SumSales:=SumSales+ItemLedgerEntry."Cost Amount (Actual)";
MESSAGE(FORMAT(ItemLedgerEntry."Cost Amount (Actual)"));
END;
UNTIL ItemLedgerEntry.NEXT=0;
Coust:=SumSales/SumQuantity;
EXIT(Coust)

I dont know what a problem ??

see my code source and tell me the solution thanks for you