Reflect accessed map does not provide a modifiable value

Context

In the process of writing a generic diff and patch algorithm I faced a problem with reflection in go.

When I'm trying to patch in a slice I have no problem, reflect.ValueOf(&slice).Elem().Index(0).CanSet() returns true. This allows me to patch anything inside the slice element, be it a slice of primitive or of structs.

Problem

However when I attempt this with a map reflect.ValueOf(&map).Elem().MapIndex(reflect.ValueOf("key")).CanSet() returns false. This prevents me from attempting to do anythnin with the content of my map.

Examples

Slice

s := []string{"a", "b", "c"}

v := reflect.ValueOf(&s).Elem()

e := v.Index(1)

println(e.String())
println(e.CanSet())
e.Set(reflect.ValueOf("d"))

for _, v := range s {
    print(v, " ")
}

output :
b
true
a d c

Map

m := map[string]string{
    "a": "1",
    "b": "2",
    "c": "3"}

mv := reflect.ValueOf(&m).Elem()
println(mv.MapIndex(reflect.ValueOf("a")).CanSet())

output:
false

How could I get a modifiable value out of a map through reflection?

Thanks for your time.