NPE when creating a new user with wicket logging at DEBUG

When creating a new user, while logging wicket at DEBUG, the following NPE occurs:

java.lang.NullPointerException
at org.hippoecm.frontend.plugins.cms.admin.users.DetachableUser.hashCode(DetachableUser.java:60)
at java.lang.Object.toString(Object.java:219)
at org.apache.wicket.model.LoadableDetachableModel.toString(LoadableDetachableModel.java:149)
at java.lang.String.valueOf(String.java:2826)
at java.lang.StringBuffer.append(StringBuffer.java:219)
at org.slf4j.helpers.MessageFormatter.deeplyAppendParameter(MessageFormatter.java:237)
at org.slf4j.helpers.MessageFormatter.arrayFormat(MessageFormatter.java:196)
at org.slf4j.helpers.MessageFormatter.format(MessageFormatter.java:138)
at org.slf4j.impl.Log4jLoggerAdapter.debug(Log4jLoggerAdapter.java:246)
at org.apache.wicket.model.LoadableDetachableModel.detach(LoadableDetachableModel.java:106)