Can't change a bigint toString

Can't change a bigint toString

I can't seem to change my "counter" variable into a string. The toString function does not appear in the options of functions available. Any ideas what I am doing wrong?

objOffer = zoho.crm.getRecordById("CustomModule1",input.vOfferId);
vOppId=objOffer.get("Opportunity_ID");
response = zoho.crm.searchRecordsByPDC("CustomModule1","Opportunity_ID",vOppId);
counter=0;
for each item in response
{
counter=(counter  +  1);
}
if( counter <  10)
{
strCounter="0" + counter;
}
info strCounter;