Error when removing user

The following error occurred when deleting a user via the global administration:

Cause

java.lang.NullPointerException
at com.atlassian.confluence.security.ContentPermission.compareTo(ContentPermission.java:197)
Stack Trace:[hide]

java.lang.NullPointerException
at com.atlassian.confluence.security.ContentPermission.compareTo(ContentPermission.java:197)
at java.util.TreeMap.put(TreeMap.java:545)
at java.util.TreeSet.add(TreeSet.java:238)
at java.util.AbstractCollection.addAll(AbstractCollection.java:305)
at java.util.TreeSet.addAll(TreeSet.java:295)
at net.sf.hibernate.collection.Set.endRead(Set.java:244)
at net.sf.hibernate.impl.SessionImpl.endLoadingCollections(SessionImpl.java:3114)
at net.sf.hibernate.impl.SessionImpl.endLoadingCollections(SessionImpl.java:3101)
at net.sf.hibernate.loader.Loader.initializeEntitiesAndCollections(Loader.java:324)
at net.sf.hibernate.loader.Loader.doQuery(Loader.java:309)