Drush returns errro "Fatal error: Can't use method return value in write context..."

When using "drush cc all", drush returns this error caused by commerc_billy_pdf submodule:

Fatal error: Can't use method return value in write context in .../sites/all/modules/commerce_billy/modules/commerce_billy_pdf/commerce_billy_pdf.module on line 213

This might popup based on PHP version used. The fix would be to store method return value inside variable and pass that to the empty() method like:

$date_arg = $wrapper->{$date_field_name}->value();
$date = !empty($date_arg) ? $date_arg : $order->created;

And the error is gone.