Using getter for an unordered_map in c++ creates an invalid read of size 8

I'm trying to use a getter for a field std::unordered_map<std::string, User *> userMap to find a user in it.

I tired doing:

   std::unordered_map<std::string, User *>::const_iterator found = getUserMap().find(userName);

but valgrind detected an invalid read of size 8. However when I did this:

std::unordered_map<std::string, User *> tmpUserMap = getUserMap();
    std::unordered_map<std::string, User *>::const_iterator found = tmpUserMap.find(userName);

The invalid read went away.

The getter is a standard getter:

std::unordered_map<std::string, User *> Session::getUserMap() const{
    return userMap;
}

If needed I can provide the valgrind data, i'm using c++11 on clion editor.