Errors with HTTP redirect Binding

Specifications for redirect binding says : 

"HTTP interactions during the message exchange MUST NOT use HTTP error status codes to indicate

failures in SAML processing, since the user agent is not a full party to the SAML protocol exchange."

If a SP receive a request with this binding and the URI indicated in the issuer element of the request is unknown, the SP can't guess the URL of the sender and then, can't send any response to it. So the only way is to send an HTTP error status... Is it a contradiction with preceding "MUST NOT" ?

Val廨ie