Struct immutable data and dict

[Patric Dexheimer via Digitalmars-d-learn]

struct Test{
    immutable size_t id;
}

Test[string] dict;
Test[] array;
void main(){
    array~=Test(1);//work
    dict["teste"] = Test(1); //fail ??
}

"Error: cannot modify struct dict["teste"] Test with immutable 
members"
Why?