C to MIPS char**

I am trying to convert the following code from C to MIPS

struct Puzzle {
    int NUM_ROWS;
    int NUM_COLS;
    char** board;
};

char floodfill (Puzzle* puzzle, char marker, int row, int col) {
    if (row < 0 || col < 0) {
        return marker;
    }
    if (row >= puzzle->NUM_ROWS || col >= puzzle->NUM_COLS) {
        return marker;
    }
    char ** board = puzzle->board;
    if (board[row][col] != ’#’) {
        return marker;
    }
    board[row][col] = marker;
    floodfill(puzzle, marker, row + 1, col + 1);

    return marker + 1;
}

I am accessing char** board by doing one load word get the address of the array of pointers out of the struct, a second load word to get the address of the array from the array of pointers, and a third load word to get the actual data.

To get board[row][col] I am adding 4 * row to where char ** board is stored, loading the data at that address, then adding col to that address.

How do I actually change the value at board[row][col] to marker? When I use sb I get an exception.

Exception occurred at PC=0x0040027c

Unaligned address in store: 0x00000023

PC=0x0040027c is this line sw $a1, 0($s3) # board[row][col] = marker

lw $t0, 8($a0)              # lw - address of the array of pointers out of the struct
lw $t0, 0($t0)              # char ** board = puzzle->board;
mul $t3, $a2, 4             # t3 = 4 * row
add $t3, $t0, $t3           # t3 = t0 + t3
lbu $s3, 0($t3)             # load the data at t3
add $s3, $s3, $a3           # add col to the $s3 address

sb $a1, 0($s3)              # board[row][col] = marker;