error while using mutex

when I try to make a map that has string as key and pthread_mutex_t as element,

    map<string, pthread_mutex_t> connectedClientsMutexes;

    pthread_mutex_t myMutex;//= PTHREAD_MUTEX_INITIALIZER;
    connectedClientsMutexes.insert(pair<string,pthread_mutex_t>(userName,myMutex));
while (1)
    {

    pthread_mutex_lock(&connectedClientsMutexes[userName]); 
     // do something here 
    }

this generates:

phase3: pthread_mutex_lock.c:312: __pthread_mutex_lock_full: Assertion `(-(e)) != 3 || !robust' failed.