Undefined array key "paymentMethod"

If I provide a wrong (test) card number (e.g. 5102589999999913, 123, 06/2026) on checkout I get the following php warning:

Warning: Undefined array key "paymentMethod" in Drupal\commerce_datatrans\Plugin\Commerce\PaymentGateway\DatatransBase->createPaymentMethod() (line 237 of modules/contrib/commerce_datatrans/src/Plugin/Commerce/PaymentGateway/DatatransBase.php).

There should be no such warning, the standard payment failure message is enough.