How to get rid of garbage characters in this program?

This program is supposed to print an input string backwards. Every single time it happens, though, I get garbage characters such as \340 or of the like. Why is it doing that? Here's my code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main()
{
    char mattsentence[51];
    mattsentence[50] = '\0';
    gets(mattsentence);
    char mask[sizeof(mattsentence)];
    int i, j;
    j = sizeof(mattsentence);
    for (i = 0; i < sizeof(mask); i++)
    {
        j = j - 1;
        mask[i] = mattsentence[j];
        printf("%c", mask[i]);
    }
    printf("\n");
}