THIRD EDIT: I now believe that this problem is due to a SOAP version mismatch (1.1 request, 1.2 response) masquerading as a namespace problem. Is it illegal to mix versions, or just bad style? Am I completely out of luck if I can't change my SOAP version or the service's?

SECOND EDIT: Clarified error message, and tried to reduce "tl;dr"-ness.

EDIT: [Link deleted, not related]

Using soapUI, I'm sending a request that starts with:

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" ...

and getting a response that starts with:

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

I know the service is getting the info, because processes down the line are working. 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.

I have tried mixing and matching the namespace prefixes and schema URLs. Changing prefixes seems to have no effect; changing URLs causes a VersionMismatch error. I have also tried to use a substitution group, but that doesn't seem to be legal.