protected function _debugExpectedFields($expectedFields = [], $missingMessage = '')
    {
        if (count($expectedFields) === 0) {
            return null;
        }

        $expectedFieldNames = [];
        foreach ((array)$expectedFields as $key => $expectedField) {
            if (is_int($key)) {
                $expectedFieldNames[] = $expectedField;
            } else {
                $expectedFieldNames[] = $key;
            }
        }

        return sprintf($missingMessage, implode(', ', $expectedFieldNames));
    }