The Bill property is not valid when DocType != DocumentType.Bill

[sgutbaplicada]

Hi, I'm receiving this error from MPOST ''The Bill property is not valid when DocType != DocumentType.Bill''
i can't replicate this situation....in wich situations you receive this error? when someone is trying to use something that's not a bill?
is it correct to solve it like this?
if (BillAcceptor.DocType == DocumentType.Bill)
{
(.....)
BillAcceptor.EscrowStack();
}
else
{
(....)
BillAcceptor.EscrowReturn();
}

thanks