anyone know why i get an index error on grid[i][j]??

n = len(grid)

for i in range(n): # go by row
    # sort row
    #print(f"unsorted row: {grid[i]}")
    row = list(grid[i])
    row.sort()
    row = ''.join(row)
    #print(f"sorted row: {row}")

    grid[i] = row
    print(f"sorted row: {grid[i]}")
    if i == 0:
        continue

    # check columns
    j = 0
    while j < n:
        print(f"i: {i}, j: {j}")
        print(f"grid[i][j]: {grid[i][j]}")

        if grid[i][j] < grid[i-1][j]: # if cur < prev -> False
            return "NO"
        j += 1

return "YES"