NPE in AbstractMailReceiver.receive()

Running in a NPE at executing some of my integration Tests.

Line 245 in AbstractMailReceiver contains following code which failed:

boolean recentFlagSupported = this.getFolder().getPermanentFlags().contains(Flags.Flag.RECENT);

.getPermanentFlags().contains is failing because of my MockMail Instance is returning null for getPermanentFlags().
This is Specification Conform which says that this Method can return null.