Setting values of nested struct fields using reflection in Go

I'm trying to create a protobuf structure out of a

type FlatMessage map[string][]byte

and I'm struggling with setting values of nested fields.

I have an

func (fm *FlatMessage) Unflatten() (pb.Message, error)

method to transform flat message into structured protobuf.

I also have a helper function called analyze which takes a struct of type pb.Message and returns a map[int]*ProtoField where:

type ProtoField struct {
    Name  string
    Type  reflect.Type
}

i.e. analyze traverses pb.Message recursively and gathers all the information I need about it. So far so good.

Now when I go key by key through my FlatMessage, the key is an encoded field number of a respective protobuf field, and I can set it using reflection, like this:

r := reflect.ValueOf(&result).Elem().FieldByName(field.Name)
if r.IsValid() {
    r.Set(scalarValue)
}

but that works only when the field.Name in question does not refer to a nested field, i.e., setting OldV1Id works fine, but attempting to set Profile.Id or, say, Destination.Address.TypeOfNumber, results in:

panic: reflect: call of reflect.Value.Set on zero Value [recovered]
    panic: reflect: call of reflect.Value.Set on zero Value

I understand that this is due to the fact that my r becomes <invalid Value> when called on a field which is a nested one, but how can I work around it?

My r is of course not valid, not addressable, and not settable. I can make a reflect.New out of it, but I can't figure out how to set that new reflect.Value in such a way that the field of my original structures becomes modified. No matter what I do the my function doesn't modify fields with nested names with this approach.

Another solution I tried is adding a Value reflect.Value to my ProtoField structure, and modifying analyze so that it appends a reflect.ValueOf(s).Field(i) where s is the top-level struct interface{} and i is its ith field. Then whenever I encounter a field that is a nested one, I call r := reflect.ValueOf(field.Value), but then the problem is I'm unable to call r.Set(scalarValue) because of incompatible types.

Any help or insight is much appreciated.