为什么将<nil>分配给Go中的接口类型是否有效？

http://play.golang.org/p/_GP3RZTh4Q

package main

import "fmt"

type TesterInterface interface{
    Yell()
}

type Tester struct{}

func (t Tester) Yell() {
    fmt.Println("HELLO")
}

func main() {
    var t TesterInterface
    t = Tester{}
    t.Yell()
    t = nil
    t.Yell()
}

我希望编译器抱怨第19行(t = nil),因为nil没有实现Yell(),但程序运行并提供以下输出:

HELLO
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0xffffffff addr=0x0 pc=0x201a9]

goroutine 1 [running]:
runtime.panic(0xef060, 0x1b3d44)
    /tmp/sandbox/go/src/pkg/runtime/panic.c:266 +0xe0
runtime.panicstring(0x1b3d44, 0x1b85)
    /tmp/sandbox/go/src/pkg/runtime/panic.c:489 +0x120
runtime.sigpanic()
    /tmp/sandbox/go/src/pkg/runtime/os_nacl.c:254 +0x80
main.main()
    /tmpfs/gosandbox-229cdcdf_329829af_705be907_f12cf4dc_ed51e32e/prog.go:20 +0xa9
runtime.main()
    /tmp/sandbox/go/src/pkg/runtime/proc.c:220 +0x1c0
runtime.goexit()
    /tmp/sandbox/go/src/pkg/runtime/proc.c:1394

goroutine 2 [syscall]:
runtime.notetsleepg(0xfeefdf88, 0x0, 0xf8475800, 0xd)
    /tmp/sandbox/go/src/pkg/runtime/lock_sema.c:254 +0xa0
runtime.MHeap_Scavenger()
    /tmp/sandbox/go/src/pkg/runtime/mheap.c:463 +0xc0
runtime.goexit()
    /tmp/sandbox/go/src/pkg/runtime/proc.c:1394
created by runtime.main
    /tmp/sandbox/go/src/pkg/runtime/proc.c:179
 [process exited with non-zero status]

Program exited.

为什么编译器认为分配nil给接口是合法的？

另外,来自Go-docs:

接口类型的变量可以使用方法集存储任何类型的值,该方法集是接口的任何超集.

"nil"如何包含一个超集的方法集TesterInterface？文档接着说

接口类型的未初始化变量的值为nil.

但这似乎与第一个陈述相矛盾.