net/http: client crash with custom RoundTrip

[rsc]

This program worked in Go 1.6 and earlier but crashes with Go 1.7.

goroutine 1 [running]:
panic(0x1d1440, 0xc82000c160)
    /Users/rsc/go/src/runtime/panic.go:500 +0x18c
net/http.(*Client).doFollowingRedirects.func1(0x2e8680, 0xc82000d0e0, 0xc82002e030, 0x1)
    /Users/rsc/go/src/net/http/client.go:446 +0x93
net/http.(*Client).doFollowingRedirects(0x2fdc00, 0xc8200d20f0, 0x226478, 0xf, 0x0, 0x0)
    /Users/rsc/go/src/net/http/client.go:489 +0x8ad
net/http.(*Client).Get(0x2fdc00, 0x20de9c, 0xf, 0x23b9, 0x1d5140, 0xc8200001a0)
    /Users/rsc/go/src/net/http/client.go:415 +0x89
main.main()
    /tmp/x.go:27 +0x38

package main

import (
    "fmt"
    "io/ioutil"
    "net/http"
    "strings"
)

var client = &http.Client{
    CheckRedirect: func(*http.Request, []*http.Request) error { return fmt.Errorf("no redirects!") },
    Transport:     new(tripper),
}

type tripper struct{}

func (tripper) RoundTrip(*http.Request) (*http.Response, error) {
    resp := &http.Response{
        StatusCode: 303,
        Header:     map[string][]string{"Location": {"http://www.example.com/"}},
        Body:       ioutil.NopCloser(strings.NewReader("")),
    }
    return resp, nil
}

func main() {
    client.Get("http://foo.com/")
}