Can some one please help me with this code snippet?

I am running this code as a script task in a business process.

string POid = Get<string>("PurchaseOrderID");
var arParts = POid.Split('_');
 
 
		int i = Convert.ToInt32(arParts[1]);
		int newid = i + 1;
		string prefix = string.Empty;
		if(newid<10){
			prefix="00000";
		}
		else if (newid<100)
		{ prefix="0000";
		}
		else if (newid<1000)
		{
			prefix="000";
		}
		else if (newid<10000)
		{
			prefix="00";
		}
		else if (newid<100000)
		{
			prefix="0";
		}
 
		string intstringid = newid.ToString();
		string name = prefix + intstringid; 
		string newPOid = "PO_" + name;
		Set("NewPurchaseOrderID", newPOid);
return true;

I have "PurchaseOrderID" and "NewPurchaseOrderID" as the parameters that I've defined for this business process. They are spelled as in the code, I've checked that.

When I try to run this process, I get the error: Object reference not set to an instance of an object.

Can't seem to find which object I have not initiated.

Any help would be appreciated.

Thanks.

-AK