C # string array assignment

I have a piece of code working with an array of strings

// 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; } 

Where the line is highlighted - when run in the studio, the compiler missed, values ​​are assigned. However, on the test site, the following error occurs, I quote:

nhandled Exception: System.FormatException: The input string was not in a correct format. at System.Number.StringToNumber (System.ReadOnlySpan 1[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.ReadOnlySpan 1 [T] s, System.Globalization.NumberStyles style, System.Globalization.NumberFormatInfo info) [0x0000a] in <04750267503a43e5929c1d1ba19daf3e> 0 at System.I.A.It32.It32.a3e5929c1d1ba19daf3e> 0 at system.I.A.I.I.I.OncErnEnlySpan 1) 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: The input string is not correct. at System.Number.StringToNumber (System.ReadOnlySpan 1[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.ReadOnlySpan 1 [T] s, System.Globalization.NumberStyles style, System.Globalization.NumberFormatInfo info) [0x0000a] in <04750267503a43e5929c1d1ba19daf3e> 0 at System.I.A.It32.It32.a3e5929c1d1ba19daf3e> 0 at system.I.A.I.I.I.OncErnEnlySpan 1) 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

Please give ideas, thank you.