Newbie - Debug error in simple class

Dear experts,

I am receiving the following error with a small practice program I am writing to test my knowledge on pointers and operator overloading

Debug Assertion Failed!
Expression: _BLOCK_TYPE_IS_VALID(pHead->nBlockUse)

Here is my class Rectangle and the main method which creates objects of it, the point at which the error occurs is before it prints the final 3 cout's (on the statement that utlizes the overloaded operator + for my Rectangle class). Why am I getting the error?

#include "stdafx.h"
#include <iostream>

using namespace std;

class Rectangle
{
private:
      int * height, * width;

public:
      Rectangle () {};
      Rectangle(int, int);
      ~Rectangle();

      int get_area(void);
      int get_height(void);
      int get_width(void);

      Rectangle operator + (Rectangle);
};

Rectangle::Rectangle(int a, int b)
{
      height = new (nothrow) int;
      width = new (nothrow) int;

      *height = a;
      *width = b;
}

Rectangle::~Rectangle()
{
      delete height;
      delete width;
}

int Rectangle::get_area(void)
{
      return get_height() * get_width();
}

int Rectangle::get_height(void)
{
      return *height;
}

int Rectangle::get_width(void)
{
      return *width;
}

Rectangle Rectangle::operator +(Rectangle rect)
{
      Rectangle retRect ((this->get_height() + rect.get_height()), (this->get_width() + rect.get_width()));
      return retRect;
}

int _tmain(int argc, _TCHAR* argv[])
{
      Rectangle rect1 (50, 4), *rect1Ptr;
      rect1Ptr = &rect1;

      cout << "rect 1 Height: " << rect1Ptr->get_height() << endl;
      cout << "rect 1 Width: " << rect1Ptr->get_width() << endl;
      cout << "rect 1 Area: " << rect1Ptr->get_area() << endl;

      cout << endl;

      Rectangle rect2 (20, 20);
      cout << "rect 2 Height: " << rect2.get_height() << endl;
      cout << "rect 2 Width: " << rect2.get_width() << endl;
      cout << "rect 2 Area: " << rect2.get_area() << endl;

      cout << endl;

      Rectangle rect3;
      rect3 = *rect1Ptr + rect2; // Error occurs on this line
      cout << "rect 3 Height: " << rect3.get_height() << endl;
      cout << "rect 3 Width: " << rect3.get_width() << endl;
      cout << "rect 3 Area: " << rect3.get_area() << endl;
      
      system("pause");
      return 0;
}