Soap header elements have to be called twice!

for the following:

<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/">
<s:Header>
<credentials xmlns="http://www.xxxx.com/Services/1.1">
<deviceProvider>Sonos</deviceProvider>
<sessionId>7d30bb0aa7ed17ad0681568e335a96333dc7ecefa944066026f850eddb77ffac</sessionId>
</credentials>
</s:Header>
<s:Body>
<getMediaMetadata xmlns="http://www.xxxx.com/Services/1.1">
<id>TRACK:2646108</id>
</getMediaMetadata>
</s:Body>
</s:Envelope>

calling this
context.getMessage().getSOAPHeader().getElementsByTagName("credentials")

throws a "An exception occurred: org.w3c.dom.DOMException"

calling it a second time returns the element org.jboss.ws.core.soap.NodeListImpl@16b722d