OpenSAML user discussion

I am using the org.opensaml.saml2.binding.encoding.HTTPSOAP11Encoder class to 
encode an ArtifactResolve request.  Everything works fine if I don't try to 
sign the request, but I'm stuck on the following error when I try to sign it. 
 I don't have any problems using similar code using the 
HTTPRedirectDeflateEncoder.

this works (same credential)
<code>
messageContext.setOutboundSAMLMessageSigningCredential(getSigningCredential());

HTTPRedirectDeflateEncoder encoder = new HTTPRedirectDeflateEncoder()
encoder.encode(messageContext);
</code>

but this breaks:

<code>
messageContext.setOutboundSAMLMessageSigningCredential(getSigningCredential());

HTTPSOAP11Encoder encoder = new HTTPSOAP11Encoder();
encoder.encode(messageContext);
</code>

with this error:

ERROR org.opensaml.xml.io.AbstractXMLObjectMarshaller - Unable to root 
namespaces of cached DOM element, 
{urn:oasis:names:tc:SAML:2.0:protocol}ArtifactResolve
org.opensaml.xml.parse.XMLParserException: Unable to resolve namespace prefix 
null found on element {http://www.w3.org/2000/09/xmldsig#}Signature
        at org.opensaml.xml.util.XMLHelper.rootNamespaces(XMLHelper.java:893)

Does anyone have any ideas why this is breaking in the 
BaseMessageEncoder.marshallMessage(BaseMessageEncoder.java:85) step?

thanks,
Bleu