Create FRE Object in delegate functions

UIAlertView* alert = [[UIAlertView alloc] initWithTitle:@"Titlte"
                                                    message:@"my message"
                                                   delegate:self
                                          cancelButtonTitle:@"OK"
                                          otherButtonTitles: nil];
[alert show];
....
- (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex
{
    NSString* string = @"123";
    FREObject asString = nil;
    const char* utf8String = string.UTF8String;
    unsigned long length = strlen( utf8String );
    FREResult res = FRENewObjectFromUTF8( length + 1, (uint8_t*) utf8String, &asString );
    NSLog(@"res = %d", res); 
}

In function clickedButtonAtIndex res = FRE_WRONG_THREAD

Why is that?