c++ - Overloading subscript operator not working as expected

Question

I have a String struct that I overloaded the subscript operator on. But it doesn't seem to work.

//my_string.h

struct String {
    char* Text;
    uint64 Length;

    char& operator[](int32 index); 
}

//my_string.cpp

char& String::operator[](int32 index) {
    ASSERT(index >= 0);
    return Text[index];
}

//main.cpp

String* test = string_create("Hello world");
char c = test[0];

Visual Studio gives me the following error:

no suitable conversion function from "String" to "char" exists