map [] operand doesn't work c++

hi my code looks something like this

class State
{
public:
    State * create(void);
};

typedef State * (*createFunc)(void);
class Registery
{
private:
   static std::map<std::string, createFunc> registery()
public:
   static void register_func(std::string key, createFunc func)
   {
       registery[key] = func;
   }
   static State * create(std::string key)
   {
       return registery[key]();
   }
};

int main()
{
   Registery::register_func("state", State::create);
   State * s = Registery::create("state");
}

the problem is that when I try to compile it I receive this error:

error: no match for ‘operator[]’ (operand types are ‘std::map< std::basic_string< char>, State* (*)()>()’ and ‘std::string {aka std::basic_string< char>}’)

as far as I can see the types match so I don't really understand why I have an error