When I have a custom type in a Map relation it thows an exception.

exception:
java.lang.NullPointerException
at java.util.Hashtable.put(Hashtable.java:514)
at org.hibernate.cfg.annotations.SimpleValueBinder.fillSimpleValue(SimpleValueBinder.java:516)
at org.hibernate.cfg.annotations.SimpleValueBinder.make(SimpleValueBinder.java:414)
at org.hibernate.cfg.annotations.MapBinder.bindKeyFromAssociationTable(MapBinder.java:305)
at org.hibernate.cfg.annotations.MapBinder.access$000(MapBinder.java:76)
at org.hibernate.cfg.annotations.MapBinder$1.secondPass(MapBinder.java:111)
at org.hibernate.cfg.CollectionSecondPass.doSecondPass(CollectionSecondPass.java:70)
at org.hibernate.cfg.Configuration.originalSecondPassCompile(Configuration.java:1697)
at org.hibernate.cfg.Configuration.secondPassCompile(Configuration.java:1426)
at org.hibernate.cfg.Configuration.buildMappings(Configuration.java:1373)
at org.hibernate.testing.junit4.BaseCoreFunctionalTestCase.afterConstructAndConfigureConfiguration(BaseCoreFunctionalTestCase.java:162)
at org.hibernate.testing.junit4.BaseCoreFunctionalTestCase.buildSessionFactory(BaseCoreFunctionalTestCase.java:126)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at org.hibernate.testing.junit4.TestClassMetadata.performCallbackInvocation(TestClassMetadata.java:205)
... 15 more

e.g. mapping:
@ElementCollection
@MapKeyType(@Type(type = "org.hibernate.test.annotations.enumerated.custom_types.FirstLetterType"))
Map<FirstLetter, String> firstLetterMap = new HashMap<FirstLetter, String>();