Problem with creating custom post type.

[fajksk]

I was trying to create post with additional fields (fields were already defined on wordpress), but those fields wasn't saved on wordpress site.
Code which I was trying to execute is
NSMutableArray *array = [NSMutableArray array];
[array addObject:@{@"firstname":@"Krzysztof"}];
[array addObject:@{@"lastname":@"Fajks"}];
data[@"custom_fields"] = array;
data[@"post_type"] = @"rfuser";
data[@"post_status"] = @"publish";
[self.wpApi publishPostWithDict:data success:nil failure:^(NSError *error) {
NSLog(@"Error %@", error);
}];

I'm not sure, what is wrong because request generated from this library seems to be correct.