Kevin,

I have made the changes you suggested and I am now getting the following error:
Object reference not set to an instance of an object.

Would I need to change the following from:

public DataSet GetInvoicesByCustomerAndCurrencyID(object customerID)
{
// code
}

to:

public DataSet GetInvoicesByCustomerAndCurrencyID(object customerID, object currencyID)
{
// code
}