Ceasar Cipher only accounts for letters. In this shift cipher, I shift all characters by a certain <shift> int

NSString+ShiftCipher.m

// Shift all characters by a certain <shift> int
-(NSString*) asciiShift:(int)shift {
    if ([self length] == 0) {
        return nil;
    }

    // no shift, return unencrypted string
    if (shift == 0) {
        return self;
    }
    
    NSMutableString* result = [NSMutableString stringWithString:self];

    for (int i = 0; i < [result length]; i++) {
        char curChar = [result characterAtIndex:i]; // NSString to ASCII int
        if (curChar == ' ') {
            continue;
        }
        char replacementChar = curChar + shift;
        [result replaceCharactersInRange:NSMakeRange(i, 1) withString:[NSString stringWithFormat:@"%c", replacementChar]];
    }
    
    // clean result if needed
    return [result stringByReplacingOccurrencesOfString:@" " withString:@""];
}