ClassCastException occurs when importing CLOB-fields using SqlEntityProcessor and SortedMapBackedCache

Using the SqlEntityProcessor with the SortedMapBackedCache as cache implementation, gives the following ClassCastException when trying to import a field of type oracle.sql.CLOB.

2014-01-02 09:32:19,143 [ERROR] [Thread-54] Exception in entity : <field-name>:java.lang.ClassCastException: oracle.sql.CLOB cannot be cast to java.lang.Comparable
at java.util.TreeMap.getEntry(TreeMap.java:325)
at java.util.TreeMap.get(TreeMap.java:255)
at org.apache.solr.handler.dataimport.SortedMapBackedCache.add(SortedMapBackedCache.java:61)
at org.apache.solr.handler.dataimport.DIHCacheSupport.populateCache(DIHCacheSupport.java:124)
at org.apache.solr.handler.dataimport.DIHCacheSupport.getSimpleCacheData(DIHCacheSupport.java:199)
at org.apache.solr.handler.dataimport.DIHCacheSupport.getCacheData(DIHCacheSupport.java:147)
at org.apache.solr.handler.dataimport.EntityProcessorBase.getNext(EntityProcessorBase.java:129)
at org.apache.solr.handler.dataimport.SqlEntityProcessor.nextRow(SqlEntityProcessor.java:75)
at org.apache.solr.handler.dataimport.EntityProcessorWrapper.nextRow(EntityProcessorWrapper.java:243)
at org.apache.solr.handler.dataimport.DocBuilder.buildDocument(DocBuilder.java:469)
at org.apache.solr.handler.dataimport.DocBuilder.buildDocument(DocBuilder.java:495)
at org.apache.solr.handler.dataimport.DocBuilder.buildDocument(DocBuilder.java:408)
at org.apache.solr.handler.dataimport.DocBuilder.doFullDump(DocBuilder.java:323)
at org.apache.solr.handler.dataimport.DocBuilder.execute(DocBuilder.java:231)
at org.apache.solr.handler.dataimport.DataImporter.doFullImport(DataImporter.java:411)
at org.apache.solr.handler.dataimport.DataImporter.runCmd(DataImporter.java:476)
at org.apache.solr.handler.dataimport.DataImporter$1.run(DataImporter.java:457)
– org.apache.solr.handler.dataimport.EntityProcessorWrapper

It seems that this occurs because the SortedMapBackedCache uses a java.util.TreeMap as the underlying cache, and TreeMap requires that all elements implements the java.lang.Comparable interface. However oracle.sql.CLOB does not implement Comparable and the import fails when the TreeMap implementation tries to cast the element to a Comparable (this occurs on line 325 in TreeMap.java - java version 1.6.0_33).