I'm new to SOAP and web services, and I'm getting an error I don't understand. Using soapUI, I'm sending the following request:

<soapenv:Envelope  
    xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/"  
    xmlns:doc="http://myproj.mycompany.com">  
  <soapenv:Header/>  
  <soapenv:Body>...

and getting this response:

<soap:Envelope  
    xmlns:soap="http://www.w3.org/2003/05/soap-envelope">
  <soap:Body>...

I know the service is getting the info, because things are happening properly down the line. However, my soapUI teststep fails. It has two active assertions: "SOAP Response" and "Not SOAP Fault." The failure marker is next to "SOAP Response," with the following message:

line -1: Element Envelope@http://www.w3.org/2003/05/soap-envelope is not a valid Envelope@http://schemas.xmlsoap.org/soap/envelope/ document or a valid substitution.

So far, I have tried modifying the URLs and namespaces of the messages to match each other, and adding the following line:

<soapenv:Envelope
    xmlns:soapenv="http://w3.org/2003/05/soap-envelope"
    substitutionGroup="http://schemas.xmlsoap.org/soap/envelope/"/>

Is this namespace mixing legal? Is my problem actually something else?