NPE adding a null key to a TreeMap with JDK7

The following tests throw NPE:

org.drools.verifier.data.VerifierDataMapsTest.testSaveVerifierComponentAndGetForAllComponentTypes
org.drools.verifier.data.VerifierDataMapsTest.testSaveVerifierComponentAndGetForAllFields
org.drools.verifier.data.VerifierDataMapsTest.testSaveVerifierComponentAndGetForAllRestrictions
org.drools.verifier.data.VerifierDataMapsTest.testSavePatternAndGet

java.lang.NullPointerException
at java.util.TreeMap.compare(TreeMap.java:1188)
at java.util.TreeMap.put(TreeMap.java:531)
at org.drools.verifier.data.VerifierDataMaps.add(VerifierDataMaps.java:162)
at org.drools.verifier.data.VerifierDataMapsTest.saveVerifierComponentAndGet(VerifierDataMapsTest.java:321)
at org.drools.verifier.data.VerifierDataMapsTest.testSaveVerifierComponentAndGetForAllComponentTypes(VerifierDataMapsTest.java:105)

All the tests in class fail with similar exception. From the test, it seems that objects added to the map are not null and their values used in compareTo() are not null either. This happens only with Sun JDK 7.