STL Container as private member. Segmentation Fault

I'm having a strange problem with a STL Container.

I have a class with a private member std::map<string, string> _environment. Why when I call _environment["name"]="john" in the class constructor (or anywhere), I get a Segmentation fault?

It should be the most common use of a STL container, shouldn't it?

Thanks!

Edit (more code):

In shell.h:

#include <string>
#include <map>

using namespace std;

class Shell {
    public:
        Shell();
        Shell(const Shell& orig){};
        virtual ~Shell(){};

    private:
        ...
        ...
        std::map<string, string> _environment;
};

In shell.cpp:

Shell::Shell() {    
    _environment["shell"] = "myshell";    
    ...
}
开发者_C百科

The segmentation fault occurs in the line _environment["shell"] = "myshell";

There is no reason for Segmentation fault. And for simplify inserting I recommend to use boost::assign library, like this:

Shell::Shell() {
    using namespace boost::assign;
    insert( _environment )( "shell", "myshell" );    
    ...
}

This is more elegantly and more effectively.