How can one implement a thread-safe wrapper to maps in Go by locking?

I'm trying to wrap a general map (with interface{} as both key and value) as in-memory key-value store that I named MemStore. But it is not thread-safe, despite my use of a sync.RWMutex to lock access to the underlying map. I did verify that it works fine when used from a single goroutine. However, just two concurrent goroutines accessing it results in panic: runtime error: invalid memory address or nil pointer dereference.

What is causing this problem, and what is the proper way to achieve thread-safety in Go? Whilst in this example, channels to a single goroutine interacting with the map would work, I am specifically looking for a solution that works with explicit locking.
File keyval.go:

package keyval

import "sync"

type MemStore struct {
    data map[interface{}]interface{}
    mutex sync.RWMutex
}

func NewMemStore() (MemStore) {
    m := MemStore{
        data: make(map[interface{}]interface{}),
        // mutex does not need initializing
    }
    return m
}

func (m MemStore) Set(key interface{}, value interface{}) (err error) {
    m.mutex.Lock()
    defer m.mutex.Unlock()

    if value != nil {
        m.data[key] = value
    } else {
        delete(m.data, key);
    }

    return nil
}

File keyval_test.go:

package keyval

import "testing"

func setN(store Store, N int, done chan<- struct{}) {
    for i := 0; i < N; i++ {
        store.Set(i, -i)
    }
    done <- struct{}{}
}

func BenchmarkMemStore(b *testing.B) {
    store := NewMemStore()
    done := make(chan struct{})
    b.ResetTimer()
    go setN(store, b.N, done)
    go setN(store, b.N, done)
    <-done
    <-done
}

Output of go test -bench .:

testing: warning: no tests to run
PASS
BenchmarkMemStore       panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x1 pc=0x80502eb]

goroutine 8 [running]:
runtime.panic(0x810f180, 0x821fc88)
        /usr/lib/go/src/pkg/runtime/panic.c:266 +0xac
github.com/pyramids/keyval.MemStore.Set(0x1852e4a0, 0x0, 0x0, 0x0, 0x0, ...)
        /home/steinb/go/src/github.com/pyramids/keyval/keyval.go:200 +0xb6
github.com/pyramids/keyval.(*MemStore).Set(0x1852efe0, 0x80f38c0, 0x19ff, 0x80f38c0, 0xffffe601, ...)
        /home/steinb/go/src/github.com/pyramids/keyval/keyval.go:1 +0xa1
github.com/pyramids/keyval.setN(0xb77b89d8, 0x1852efe0, 0x2710, 0x1851c090)
        /home/steinb/go/src/github.com/pyramids/keyval/memstore_test.go:69 +0x5b
created by github.com/pyramids/keyval.BenchmarkMemStore
        /home/steinb/go/src/github.com/pyramids/keyval/memstore_test.go:89 +0x116

goroutine 1 [chan receive]:
testing.(*B).run(0x1854a000, 0x3, 0xb76cdeb4, 0x1, 0x1, ...)
        /usr/lib/go/src/pkg/testing/benchmark.go:171 +0x4b
testing.RunBenchmarks(0x814d840, 0x821c828, 0x1, 0x1)
        /usr/lib/go/src/pkg/testing/benchmark.go:303 +0x464
testing.Main(0x814d840, 0x8222220, 0x0, 0x0, 0x821c828, ...)
        /usr/lib/go/src/pkg/testing/testing.go:411 +0x151
main.main()
        github.com/pyramids/keyval/_test/_testmain.go:47 +0x83

goroutine 3 [chan receive]:
github.com/pyramids/keyval.BenchmarkMemStore(0x1854a000)
        /home/steinb/go/src/github.com/pyramids/keyval/memstore_test.go:91 +0x188
testing.(*B).runN(0x1854a000, 0x2710)
        /usr/lib/go/src/pkg/testing/benchmark.go:119 +0x7a
testing.(*B).launch(0x1854a000)
        /usr/lib/go/src/pkg/testing/benchmark.go:207 +0x12c
created by testing.(*B).run
        /usr/lib/go/src/pkg/testing/benchmark.go:170 +0x32

goroutine 9 [runnable]:
sync.(*RWMutex).Lock(0x18582a64)
        /usr/lib/go/src/pkg/sync/rwmutex.go:72
github.com/pyramids/keyval.MemStore.Set(0x1852e4a0, 0x0, 0x0, 0x0, 0x0, ...)
        /home/steinb/go/src/github.com/pyramids/keyval/keyval.go:179 +0x5d
github.com/pyramids/keyval.(*MemStore).Set(0x1852efc0, 0x80f38c0, 0x2302, 0x80f38c0, 0xffffdcfe, ...)
        /home/steinb/go/src/github.com/pyramids/keyval/keyval.go:1 +0xa1
github.com/pyramids/keyval.setN(0xb77b89d8, 0x1852efc0, 0x2710, 0x1851c090)
        /home/steinb/go/src/github.com/pyramids/keyval/memstore_test.go:69 +0x5b
created by github.com/pyramids/keyval.BenchmarkMemStore
        /home/steinb/go/src/github.com/pyramids/keyval/memstore_test.go:90 +0x172
exit status 2
FAIL    github.com/pyramids/keyval      0.056s

> Effective Go
>
> Pointers vs. Values
>
> The rule about pointers vs. values for receivers is that value methods
> can be invoked on pointers and values, but pointer methods can only be
> invoked on pointers. This is because pointer methods can modify the
> receiver; invoking them on a copy of the value would cause those
> modifications to be discarded.

To visibly modify the MemStore struct variable mutex field, use a pointer receiver. You are modifying a copy, which is invisible to other go routines. For example,

File keyval.go:

package keyval

import "sync"

type MemStore struct {
	data  map[interface{}]interface{}
	mutex sync.RWMutex
}

func NewMemStore() *MemStore {
	m := &MemStore{
		data: make(map[interface{}]interface{}),
		// mutex does not need initializing
	}
	return m
}

func (m *MemStore) Set(key interface{}, value interface{}) (err error) {
	m.mutex.Lock()
	defer m.mutex.Unlock()

	if value != nil {
		m.data[key] = value
	} else {
		delete(m.data, key)
	}

	return nil
}

File keyval_test.go:

package keyval

import "testing"

func setN(store *MemStore, N int, done chan<- struct{}) {
	for i := 0; i < N; i++ {
		store.Set(i, -i)
	}
	done <- struct{}{}
}

func BenchmarkMemStore(b *testing.B) {
	store := NewMemStore()
	done := make(chan struct{})
	b.ResetTimer()
	go setN(store, b.N, done)
	go setN(store, b.N, done)
	<-done
	<-done
}

Benchmark:

$ go test -v -bench=.
testing: warning: no tests to run
PASS
BenchmarkMemStore	 1000000	      1244 ns/op
ok  	so/test	1.275s
$