Vector iterator incompatible ? Same vector

I have the following code

 vector<Interval> insert(vector<Interval>& intervals, Interval newInterval) 
    {
        vector<Interval> res;
        vector<Interval>::iterator it;
        for (it = intervals.begin(); it != intervals.end(); it++) 
        {
            if (newInterval.start < (*it).start) 
            {
                intervals.insert(it, newInterval);
                break;
            }
        }
        if (it == intervals.end())  //---->vector iterator incompatible
        {
            intervals.insert(it, newInterval); 
        }
}

I am getting an error of vector iterator incompatible at the statement

    if (it == intervals.end())  //---->vector iterator incompatible
    {
        intervals.insert(it, newInterval); 
    }

Can anyone please clarify why this is happening and how I can fix this ?