hash_set and std::string

[imi123_1999]

Friends, i am doing something like:

using namespace std;

hash_set<string> myhashstring;

myhashstring.insert("USA");


As such program is giving me error that:

(38): error C2440: 'type cast' : cannot convert from 'const std::string' to 'size_t'


But if i use "set" instead of "hash_set" the program works normally. Any one guess whats the problem ???