how can I return a 'null' smart pointer?

Hi,

Can you please tell me how can I return a 'null' smart pointer?

typedef shared_ptr<MyA> MyAPtr;
typedef vector<MyAPtr>  MyAVector;

For example,I have this code, which return NULL in an error situation:

MyAPtr myfunction(MyAVector& v, int size) {
      if (index < v.size()) {
            return v[index];
       } else {
          // how can I return NULL here?

      }

}

How can I achieve that when I use boost smart pointer?

Thank you for any tip.