error demo

demo.go

package main

import (
	"crypto/sha256"
	"fmt"
	"hash"
)

type Signer struct {
	hash.Hash
}

func (s Signer) InitHash() {
	s.Hash = sha256.New()
}

func main() {

	s := new(Signer)
	s.InitHash()
	digest := s.Sum([]byte("hello world"))

	fmt.Printf("digest: %x\n", digest)
}

demo.out

Benjamins-MacBook-Pro:test mastahyeti$ go run ./foo.go
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x38 pc=0x2139]

goroutine 1 [running]:
panic(0xdf700, 0xc82000a0c0)
	/usr/local/Cellar/go/1.6.1/libexec/src/runtime/panic.go:464 +0x3e6
main.main()
	/Users/mastahyeti/Projects/go/src/test/foo.go:21 +0xc9
exit status 2