Xcode warning “Property access results unused - getters should not be used for side effects”

I'm getting this warning when I'm calling a local routine.

My code is this:

-(void)nextLetter {
    // NSLog(@"%s", __FUNCTION__);
    currentLetter ++;
    if(currentLetter > (letters.count - 1))
    {
        currentLetter = 0;
    }
    self.fetchLetter;
}

I'm getting the warning on the self.fetchLetter statement.

That routine looks like this:

- (void)fetchLetter {
    // NSLog(@"%s", __FUNCTION__);
    NSString *wantedLetter = [[letters objectAtIndex: currentLetter] objectForKey: @"langLetter"];

    NSString *wantedUpperCase = [[letters objectAtIndex: currentLetter] objectForKey: @"upperCase"];    


.....   
}

I prefer to fix warning messages, is there a better way to write this?

Thanks!