WS-Security signatures: Id not found

I'm using soapUI 3.6.1, on Snow Leopard (java version "1.6.0_20").

I set up an outgoing WS-Security configuration with two tabs (timestamp and signature). The signature tab has two parts, with all the fields populated:

Body - Body - http://schemas.xmlsoap.org/soap/envelope/ - Element
Timestamp - Timestamp - http://docs.oasis-open.org/wss/2004/01/ ... ty-1.0.xsd - Element

When sending a request with this WSS configuration assigned, I receive the following exception and the requests go out unsigned:

Fri Oct 29 13:10:08 BST 2010:ERROR:org.apache.ws.security.WSSecurityException: Signature creation failed; nested exception is: 
 org.apache.xml.security.signature.ReferenceNotInitializedException: Id not found
Original Exception was org.apache.xml.security.signature.ReferenceNotInitializedException: Id not found
Original Exception was org.apache.xml.security.signature.ReferenceNotInitializedException: Id not found
Original Exception was org.apache.xml.security.utils.resolver.ResourceResolverException: Id not found
   org.apache.ws.security.WSSecurityException: Signature creation failed; nested exception is: 
    org.apache.xml.security.signature.ReferenceNotInitializedException: Id not found
   Original Exception was org.apache.xml.security.signature.ReferenceNotInitializedException: Id not found
   Original Exception was org.apache.xml.security.signature.ReferenceNotInitializedException: Id not found
   Original Exception was org.apache.xml.security.utils.resolver.ResourceResolverException: Id not found
    at org.apache.ws.security.message.WSSecSignature.computeSignature(WSSecSignature.java:717)
    at org.apache.ws.security.message.WSSecSignature.build(WSSecSignature.java:780)
    at com.eviware.soapui.impl.wsdl.support.wss.entries.AddSignatureEntry.process(AddSignatureEntry.java:177)
    at com.eviware.soapui.impl.wsdl.support.wss.OutgoingWss.processOutgoing(OutgoingWss.java:157)
    at com.eviware.soapui.impl.wsdl.mock.WsdlMockResponse.execute(WsdlMockResponse.java:302)
    at com.eviware.soapui.impl.wsdl.mock.WsdlMockOperation.dispatchRequest(WsdlMockOperation.java:259)
    at com.eviware.soapui.impl.wsdl.mock.WsdlMockRunner.dispatchPostRequest(WsdlMockRunner.java:300)
    at com.eviware.soapui.impl.wsdl.mock.WsdlMockRunner.dispatchRequest(WsdlMockRunner.java:384)
    at com.eviware.soapui.monitor.JettyMockEngine$ServerHandler.handle(JettyMockEngine.java:701)
    at org.mortbay.jetty.handler.HandlerCollection.handle(HandlerCollection.java:114)
    at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
    at org.mortbay.jetty.Server.handle(Server.java:326)
    at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542)
    at org.mortbay.jetty.HttpConnection$RequestHandler.content(HttpConnection.java:938)
    at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:755)
    at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:218)
    at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404)
    at org.mortbay.jetty.bio.SocketConnector$Connection.run(SocketConnector.java:228)
    at org.mortbay.jetty.security.SslSocketConnector$SslConnection.run(SslSocketConnector.java:680)
    at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
    at java.lang.Thread.run(Thread.java:637)
   Caused by: org.apache.xml.security.signature.ReferenceNotInitializedException: Id not found
   Original Exception was org.apache.xml.security.signature.ReferenceNotInitializedException: Id not found
   Original Exception was org.apache.xml.security.signature.ReferenceNotInitializedException: Id not found
   Original Exception was org.apache.xml.security.utils.resolver.ResourceResolverException: Id not found
    at org.apache.xml.security.signature.Reference.calculateDigest(Unknown Source)
    at org.apache.xml.security.signature.Reference.generateDigestValue(Unknown Source)
    at org.apache.xml.security.signature.Manifest.generateDigestValues(Unknown Source)
    at org.apache.xml.security.signature.XMLSignature.sign(Unknown Source)
    at org.apache.ws.security.message.WSSecSignature.computeSignature(WSSecSignature.java:713)
    ... 21 more

Could this be a bug?