upstream sent invalid header while reading response

Hello. I've got backend (fapws python server).
There is a problem when backend sets field expires= in Set-Cookie

2015/06/17 08:59:22 [debug] 27687#0: *8 http proxy header: "Date: Wed, 17 Jun 2015 05:59:22 GMT"
2015/06/17 08:59:22 [debug] 27687#0: *8 http proxy header: "P3P: CP="NOI ADM DEV COM NAV OUR STP""
2015/06/17 08:59:22 [debug] 27687#0: *8 http proxy header: "Content-type: text/html"
2015/06/17 08:59:22 [debug] 27687#0: *8 http proxy header: "Server: fapws3/0.11"
2015/06/17 08:59:22 [debug] 27687#0: *8 http proxy header: "Set-Cookie: sid=50158736; Domain=.domain.ru; expires=Thu, 02-Jul-2015 05:59:22 GMT; Path=/"
2015/06/17 08:59:22 [error] 27687#0: *8 upstream sent invalid header while reading response header from upstream, client: 217.15.199.73, server: t.domain.ru, request: "GET /557/1434520761446.js HTTP/1.1", upstream: "http://127.0.0.1:8802/557/1434520761446.js", host: "t.domain.ru", referrer: "http://film4ik.ru/"

If there is no expires= in Set-Cookie, all is OK:
2015/06/17 09:04:53 [debug] 30261#0: *160310 http proxy header: "Date: Wed, 17 Jun 2015 06:04:53 GMT"
2015/06/17 09:04:53 [debug] 30261#0: *160310 http proxy header: "P3P: CP="NOI ADM DEV COM NAV OUR STP""
2015/06/17 09:04:53 [debug] 30261#0: *160310 http proxy header: "Content-type: text/html"
2015/06/17 09:04:53 [debug] 30261#0: *160310 http proxy header: "Server: fapws3/0.11"
2015/06/17 09:04:53 [debug] 30261#0: *160310 http proxy header: "Set-Cookie: sid=50175274; Domain=.domain.ru; Path=/"
2015/06/17 09:04:53 [debug] 30261#0: *160310 http proxy header done

Format expires "Thu, 02-Jul-2015 05:59:22 GMT" is valid, we've looked in RFC. What can be a reason of the issue?