Epicor Function in BPM Code

Mark, I have a function with these input params:

Would my function call be this then?

int PO = 73071;
int POLine = 1;
decimal newdocunitprice = Convert.ToDecimal(0.69);
this.InvokeFunction("UpdatePO","UpdateApprovedPO",Tuple.Create(PO),Tuple.Create(POLine),Tuple.Create(newdocunitprice));

I get an error like this when I do that:

Description: BPM runtime caught an unexpected exception of ‘InvalidCastException’ type.
See more info in the Inner Exception section of Exception Details.
Program: efx.updatepo.dll
Method: AdapterRun
Original Exception Type: InvalidCastException
Framework Method: A001_CustomCodeAction
Framework Line Number: 0
Framework Column Number: 0
Framework Source: A001_CustomCodeAction at offset 889 in file:line:column :0:0