AX 12 Joining PurchTable and HcmWorker on PurchTable.WorkerPurchPlacer == HcmWorker.PersonnelNumber -

Incompatible operands for this operator.

WorkerPurchPlacer is a BigInt and PersonnelNumber is a string - in my select how do I convert and compare these fields?

select firstonly purchTable where purchTable.PurchId == paramPONum
join hcmWorker
where purchTable.WorkerPurchPlacer == hcmWorker.PersonnelNumber;