map/set iterators incompatible - what might be causing it?

[cltx]

I recently started programming in c++ with visual studio 2010, my current projekt (or solution) compiles without errors. Then I start the program and then does the program crach with the following error.

Debug Assertion Failed!
...
Expression: map/set iterators incompatible

I have tried to understand why, and what is causing it but no success. Do anyone know what might be causing it? Program crach when this code run

for(set<Square*>::iterator itv; itv != vv.end(); itv++ )
// think the problem is at 'itv != vv.end()' 

    set<Square*> set1;
    map<Square*, set<Square*>> map_set;
    for(int i=0;i<9;i++){
        for(int j=0;j<9;j++){
            Square* sq = sqs[i][j];
            set<Square*> un_v = sq->getNeighbours();
            map_set.insert(pair<Square*, set<Square*>>(sq, un_v));
        }
    }
    typedef map<Square*, set<Square*>>::iterator it_map;
    for(set<Square*>::iterator it_m = set1.begin(); it_m != set1.end(); it_m++){
        Square* sq1 = (*it_m); // square 1
        for(it_map it = map_set.begin();it != map_set.end(); it++){
            set<Square*> vv = it->second;
            set<Square*> v;// = set<Square*>(vv.begin(), vv.end());
            for(set<Square*>::iterator itv; itv != vv.end(); itv++ ){ // error here <------
                v.insert(new Square(**itv));
            }
            cout << "DEBUGG MSG" << endl; // never printed
        }
    }