Two warnings from commerceredsys/sermepa

Problem/Motivation

Warning: Invalid argument supplied for foreach() en CommerceRedsys\Payment\Sermepa->setPaymentMethod() (linea 1292 de /public_html/vendor/commerceredsys/sermepa/src/Sermepa.php).

Warning: str_split() expects parameter 1 to be string, array given en CommerceRedsys\Payment\Sermepa->setPaymentMethod() (linea 1292 de /public_html/vendor/commerceredsys/sermepa/src/Sermepa.php).

Steps to reproduce

1. Using Sermepa in test mode by now.
2. Create an order and select Sermepa payment gateway to pay it.
3. On Review pane, click on "Pay and complete purchase" to go Redsys platform to proceed with payment.
4. Review your dblog

Tried with clean D8 commerce demo / kickstart and my own dev project with identical results.