RunTime Error : map/set iterators incompatible

I have a runtime error "map/set iterators incompatible" at line 8.

void Manager::Simulate(Military* military, Shalishut* shalishut,char* args[]){
    Simulation* simulation = Simulation::GetInstance();
    Time* time = Time::GetInstance();

    multimap<int,Task*>::iterator itTasks;
    itTasks = simulation->GetTasks().begin();
    while(itTasks != simulation->GetTasks().end()){
      while (itTasks->second->GetTimeStamp() == time->GetTime()){ /*line 8 - ERROR*/
            TaskExecute(itTasks->second,military,shalishut,args);
            itTasks++;
        }
        // Unit take car of vehicles
        time->TimeIncrease();
    }

}

Simulation is declared as a multimap<int,Task*>. What is the problem?