assertion error in visual studio. palindrome program

int is_palindrome(MY_STRING palindrome);


int main(int argc, char* argv[])
{


char ch = 1;

MY_STRING palindrome = my_string_init_default();


while (1) {

    ///this puts in the line from the keyboard input to the data structure      character by character
    ch = getc(stdin);
    if (ch == '\n' || ch == EOF)
        break;
        palindrome->push_back(palindrome, ch);



    }



if (is_palindrome(palindrome)) {

    printf("Yes \n");

}
else {
    printf("No \n");
}




return 0;
}



int is_palindrome(MY_STRING palindrome) {
int i,flag;
MY_STRING temp = my_string_init_default();
MY_STRING reversed = my_string_init_default();
int size = palindrome->get_size(palindrome);

//removes all non-alphabetical characters

for (i = 0; i <= size-1; i++){
    if (isalpha(palindrome->at(palindrome, i))) {
        temp->push_back(temp, tolower(palindrome->at(palindrome,i)));

    }

}

size = temp->get_size(temp);

for (i = size - 1; i >= 0; i--) {

    reversed->push_back(reversed, temp->at(temp, i));
}

/checks to see if the reversed string above is equal to the original

for (flag = 1, i = 0; i < size; i++)
{
    if (reversed->at(reversed,i) != temp->at(temp,i))
        flag = 0;
}

if (flag == 0) {
    return 1;
}

return 0;

}

the data structure am using MY_sTRING is assumed to work just fine. is there a problem for any part of the code am using. I think its one of the loops or one of the conditional statements

am getting this assertion c>=0 and c<=255. I don't exactly understand what that error means.