Creating a story using the API reports "invalid user"

Even trying this curl command, I get the same error:

curl -H "Token: TOKEN" -H "Content-type: application/xml" -X POST -d "<story>
<story_type>feature</story_type>
<description>API test </description>
<name>API test</name>
<requested_by>Matthias</requested_by>
</story>" http://www.pivotaltracker.com/service...

Here is the debug output when using Net::HTTP

opening connection to www.pivotaltracker.com...
opened
<-><-> "HTTP/1.1 200 OK\r\n"
-> "Server: nginx/0.6.26\r\n"
-> "Date: Tue, 23 Dec 2008 10:23:30 GMT\r\n"
-> "Content-Type: application/xml; charset=utf-8\r\n"
-> "Connection: close\r\n"
-> "Set-Cookie: pivotal_t2_session_id=SESSION_ID; path=/\r\n"
-> "Status: 200 OK\r\n"
-> "X-Runtime: 0.04800\r\n"
-> "ETag: \"58eeafcd6defd1b9593eaab1f4b0f36e\"\r\n"
-> "Cache-Control: private, max-age=0, must-revalidate\r\n"
-> "Content-Length: 209\r\n"
#<net::httpok:0x1245b48>
<?xml version="1.0" encoding="UTF-8"?>
<response>
<errors>
<error>The provided requested_by user 'Produktmanagement' is not a valid member of the project.</error>
</errors>
</response>
-> "\r\n"
reading 209 bytes...
-> "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<response success="\"false\">\n <errors>\n <error>The provided requested_by user 'Produktmanagement' is not a valid member of the project.</error>\n </errors>\n</response>\n"
read 209 bytes
Conn close</net::httpok:0x1245b48>

I never tried again but it was with v1 of your API - so I guess you can close this one :-)