Error message when call signature change is not explicit enough.

[hartym]

bonobo.errors.UnrecoverableTypeError  Input type changed between calls to <function average_exchange_rates at 0x109ba6268>.
                                        Got Bag(source_currencykey='AED', reporting_currencykey='USD', monthkey='2018M06', effective_start_date=datetime.date(2018, 1, 1), avg=Decimal('0.2722528968396083500000000')) which is not of type <class 'tuple'>.

Should say "which is not of type tuple of length 0, like the previous call was", to me more intelligible. Or something in the line of this.