getHeadersByActor returns invalid element?

[Kent Tong]

Hi,
I'm working on a handler that verifies XML signatures. I use the
following code to get the XML signature element:
SOAPEnvelope env =
   (SOAPEnvelope) msgContext.getMessage().getSOAPPart().getEnvelope();
Document doc = env.getAsDocument();
doc.getElementsByTagNameNS("*", "Signature").item(0);
It is working fine. But if I change the code to:
SOAPEnvelope env =
   (SOAPEnvelope) msgContext.getMessage().getSOAPPart().getEnvelope();
ArrayList actors = ...;
SOAPHeaderElement soapSignatureHeader =
   (SOAPHeaderElement) env.getHeadersByActor(actors).get(0);
soapSignatureHeader.getAsDOM().getElementsByTagNameNS(
   "*", "Signature").item(0);
Then the checkSignatureValue method will return false. Another
strange behavior is that checkSignatureValue out a success
message '- Verification successful for URI "#Body"' to the log.
What's wrong?
TIA!