Coping Data from One Field to another in same module

I had setup a created a custom function like below but it doesn't work, can you please help

void GrnNoUpdate (int PoID, string GRNNo ) Edit Arguments

PoIDStr = input.PoID.toString() ;

mp = map() ;

mp.put("GRN No.",input.GRNNo);

updateResp = zoho.crm.updateRecord("PurchaseOrders" ,PoIDStr ,mp ) ;

I have a PO number filed which in some cases I need this to be exactly as "GRN No.", but this function won't work, i copied this with another function which is working fine from one module coping to another , but within the same module I wasn't able to make it work, can anyone please help.

Thank youl