Index: openjpa-kernel/src/main/java/org/apache/openjpa/util/ImplHelper.java
===================================================================
--- openjpa-kernel/src/main/java/org/apache/openjpa/util/ImplHelper.java	(revision 556924)
+++ openjpa-kernel/src/main/java/org/apache/openjpa/util/ImplHelper.java	(working copy)
@@ -214,7 +214,7 @@
         Boolean isAssignable = null;
         Map assignableTo = (Map) _assignableTypes.get(from);
         if (assignableTo == null) { // "to" cache doesn't exist, so create it...
-            assignableTo = new ConcurrentHashMap();
+            assignableTo = new ConcurrentReferenceHashMap(ReferenceMap.WEAK, ReferenceMap.HARD);
             _assignableTypes.put(from, assignableTo);
         } else { // "to" cache exists...
             isAssignable = (Boolean) assignableTo.get(to);