mat.hpp assert error at line 554

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I have a loop and in the 240th step bgrPixel = frame.at<vec3b>(++rowCoord, ++colCoord); throws assert erroror at mat.hpp line 554

for (int rowCoord = 1; rowCoord <
> rowheight - 1; rowCoord++) for (int
> colCoord = 1; colCoord < colwidth - 1;
> colCoord++) { ///////////// //// /////
>                     frame.at<Vec3b>(Point(rowCoord,
> colCoord)) = black;
> 
>             }
>             if (counter == 239)
>             {
>                 int o = 0;
>             }
>             c
>             cout << counter << endl;
>             counter++;
>         }