Soap 4.0.1 - signature configuration issue

Hi,

Ws security set up we have done for Soap UI 3.0.1 doesn't seem to work for 4.0.1. Exception stack trace is given below. Can some tell me the extra configuration step that we need to do.

Mon Feb 27 16:10:42 GMT+05:30 2012:ERROR:org.apache.ws.security.WSSecurityException: Signature creation failed; nested exception is:
java.lang.NullPointerException
org.apache.ws.security.WSSecurityException: Signature creation failed; nested exception is:
java.lang.NullPointerException
at org.apache.ws.security.message.WSSecSignature.computeSignature(WSSecSignature.java:721)
at org.apache.ws.security.message.WSSecSignature.build(WSSecSignature.java:780)
at com.eviware.soapui.impl.wsdl.support.wss.entries.AddSignatureEntry.process(AddSignatureEntry.java:200)
at com.eviware.soapui.impl.wsdl.support.wss.OutgoingWss.processOutgoing(OutgoingWss.java:157)
at com.eviware.soapui.impl.wsdl.submit.filters.WssRequestFilter.filterWsdlRequest(WssRequestFilter.java:58)
at com.eviware.soapui.impl.wsdl.submit.filters.AbstractRequestFilter.filterAbstractHttpRequest(AbstractRequestFilter.java:37)
at com.eviware.soapui.impl.wsdl.submit.filters.AbstractRequestFilter.filterRequest(AbstractRequestFilter.java:31)
at com.eviware.soapui.impl.wsdl.submit.transports.http.HttpClientRequestTransport.sendRequest(HttpClientRequestTransport.java:133)
at com.eviware.soapui.impl.wsdl.WsdlSubmit.run(WsdlSubmit.java:123)
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
at java.util.concurrent.FutureTask$Sync.innerRun(Unknown Source)
at java.util.concurrent.FutureTask.run(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.NullPointerException
at org.apache.ws.security.components.crypto.CryptoBase.getPrivateKey(CryptoBase.java:216)
at org.apache.ws.security.message.WSSecSignature.computeSignature(WSSecSignature.java:713)