C# строковый массив присвоение

У меня есть кусок кода, работа со с массивом строк

    // Complete the cavityMap function below.
static string[] cavityMap(string[] grid)
{
    var probablymax = 0;
    string symbolMax = "X";
    var count_arr = grid.Length - 1;
    for (int i = 0; i <= count_arr; i++)
    {
        if (i != 0 && i != count_arr &&
          int.Parse(grid[i]) > int.Parse(grid[0]) &&
            int.Parse(grid[i]) > int.Parse(grid[count_arr]))
        {
            probablymax = i;
            var comparerer = int.Parse(grid[probablymax]);
            if (int.Parse(grid[i + 1]) < comparerer && int.Parse(grid[i - 1]) < comparerer)
            {
                probablymax = i ;
                **grid[probablymax] = symbolMax;**
            }
        }

    }      
    return grid;
}

Там, где выделена строка - при запуске в студии, компилятор пропустил, значения присвоено. Однако на тестовом сайте, происходит ошибка следующая, привожу:

nhandled Exception: System.FormatException: Input string was not in a correct format. at System.Number.StringToNumber (System.ReadOnlySpan1[T] str, System.Globalization.NumberStyles options, System.Number+NumberBuffer& number, System.Globalization.NumberFormatInfo info, System.Boolean parseDecimal) [0x00045] in <04750267503a43e5929c1d1ba19daf3e>:0 at System.Number.ParseInt32 (System.ReadOnlySpan1[T] s, System.Globalization.NumberStyles style, System.Globalization.NumberFormatInfo info) [0x0000a] in <04750267503a43e5929c1d1ba19daf3e>:0 at System.Int32.Parse (System.String s) [0x00016] in <04750267503a43e5929c1d1ba19daf3e>:0 at Solution.cavityMap (System.String[] grid) [0x00071] in solution.cs:33 at Solution.Main (System.String[] args) [0x00045] in solution.cs:60 [ERROR] FATAL UNHANDLED EXCEPTION: System.FormatException: Input string was not in a correct format. at System.Number.StringToNumber (System.ReadOnlySpan1[T] str, System.Globalization.NumberStyles options, System.Number+NumberBuffer& number, System.Globalization.NumberFormatInfo info, System.Boolean parseDecimal) [0x00045] in <04750267503a43e5929c1d1ba19daf3e>:0 at System.Number.ParseInt32 (System.ReadOnlySpan1[T] s, System.Globalization.NumberStyles style, System.Globalization.NumberFormatInfo info) [0x0000a] in <04750267503a43e5929c1d1ba19daf3e>:0 at System.Int32.Parse (System.String s) [0x00016] in <04750267503a43e5929c1d1ba19daf3e>:0 at Solution.cavityMap (System.String[] grid) [0x00071] in solution.cs:33 at Solution.Main (System.String[] args) [0x00045] in solution.cs:60

Прошу дать идеи, спасибо