Overriding DefaultSimilarity Cast Exception 4.5.1

[Summer Shire]

Hi all,

I am using Solr and Lucene 4.5.1
and I have a Payload Similarity that extends DefaultSimilarity

my schema.xml specifies the default Similarity tag
<similarity class="<mypackage>.PayloadSimilarity"/>

/** my similarity class**/
public class PayloadSimilarity extends DefaultSimilarity {

    public PayloadSimilarity(){

    }

    @Override
    public float scorePayload(int doc, int start, int end, BytesRef payload) {
        if (payload != null) {
          return PayloadHelper.decodeFloat(payload.bytes, payload.offset);
        } else {
          return 1.0f;
        }
      }

}

there are no compile errors but I get runtime exception

Caused by: java.lang.ClassCastException: <mypackage>.PayloadSimilarity cannot 
be cast to org.apache.lucene.search.similarities.Similarity
        at 
org.apache.solr.schema.IndexSchema$1.getSimilarity(IndexSchema.java:929)
        at 
org.apache.solr.schema.IndexSchema.getSimilarity(IndexSchema.java:257)
        at 
org.apache.solr.update.SolrIndexConfig.toIndexWriterConfig(SolrIndexConfig.java:219)
        at 
org.apache.solr.update.SolrIndexWriter.<init>(SolrIndexWriter.java:77)
        at 
org.apache.solr.update.SolrIndexWriter.create(SolrIndexWriter.java:64)
        at 
org.apache.solr.update.DefaultSolrCoreState.createMainIndexWriter(DefaultSolrCoreState.java:267)
        at 
org.apache.solr.update.DefaultSolrCoreState.getIndexWriter(DefaultSolrCoreState.java:110)
        at org.apache.solr.core.SolrCore.openNewSearcher(SolrCore.java:1440)



Why would this be happening. if Solr was not able to find my PayloadSimilarity 
class then the error message would have been different.

any inputs?

thanks,

Summer