I'm having trouble with the following:

```
   interface MyType {
     [key: number]: number;
   }

   function myfunc<MyType>(map: MyType, key: keyof MyType) {
     map[key] = 4; // Error: type '4' is not assignable to type MyType[keyof MyType]
   }

```

I'm not sure what I need to do to make it safe to assign something to map[key]?