Lockbits function "triggers breakpoint"

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C++

Win32

GDI+

Bitmap *pbmp     //global variable

//trying to make a buffer for the constructor, but it doesn't work as expected
byte *bytes = new byte[1000];
	for (size_t i = 0; i < 1000; i++)
	{
		bytes[i] = 255;
	}
	Bitmap bmp(100, 100, 500, PixelFormat32bppARGB, bytes);

Here is how i'm creating a bitmap at runtime, and since I need to change little portions of it i'm using this function inside WM_PAINT :

void LockBits()
{
	UINT* pixels;
	// Lock a 50xs30 rectangular portion of the bitmap for writing.
	BitmapData bitmapData;
	Rect rect(20, 10, 50, 60);
			
	pbmp->LockBits( &rect, ImageLockModeWrite, PixelFormat32bppARGB, &bitmapData);

	// Write to the temporary buffer provided by LockBits.
	pixels = (UINT*)bitmapData.Scan0;

	for (UINT row = 0; row < 60; ++row)
		for (UINT col = 0; col < 50; ++col)
		{
			pixels[row * bitmapData.Stride / 4 + col] = 0xff00ff00;
		}

	// Commit the changes, and unlock the 50x30 portion of the bitmap.  
	pbmp->UnlockBits(&bitmapData);
}


//********************************************************************************
case WM_PAINT:
{
    LockBits();
    Graphics grap(GetDC(hwnd));
    grap.DrawImage(&(*pbmp), 150, 100);
}
break;

As soon as I run the program, it works (display the changed bitmap) but immediately after "Myprogram.exe has triggered a breakpoint." and then points to the line inside WinMain -DispatchMessage(&msg);-


EDIT: It appears that I didn't gave a big enough buffer to the bitmap constructor, setting it to 40000 does the trick. But isn't it a way too big number for a 100x100 bitmap ? How should I calculate buffer size ? width * height * 4 (since pixel format is 32bits) ?