Why does Realm throw an exception when asking for `size == 4`?

My subclasses RLMObject

look like this:

@interface ImageRealm : RLMObject

@property NSString *httpsURL;
@property NSNumber<RLMInt> *size;

@end

RLM_ARRAY_TYPE(ImageRealm)

@interface PhotoRealm : RLMObject

@property NSNumber<RLMInt> *photoID;
@property RLMArray<ImageRealm *><ImageRealm> *differentSizeImages;  

- (id)initWithMantleModel:(PhotoModel *)photoModel;

@end

      

        
        
        
      

    

I want to filter an array PhotoRealm

differentSizeImages

to get a specific one ImageRealm

. I've tried using the following code:

PhotoRealm *photo = self.array[indexPath.row];
NSString *filter = @"size == 4";
ImageRealm *pecificImage = [[photo.differentSizeImages objectsWhere:filter] firstObject];

      

        
        
        
      

    

Here it is self.array

initialized like this:

self.array = [PhotoRealm allObjects];

      

        
        
        
      

    

The code throws an exception:

2017-03-24 03: 33: 36.891 project_name [46277: 3636358] *** Application terminated due to uncaught exception "Invalid predicate expressions", reason: "Predictive expressions must compare a key path and another key path or a constant value

Update: Before I added the property size

, I was able to do the following (since I only had one image size):

ImageRealm *image = [photo.differentSizeImages objectAtIndex:0];

      

        
        
        
      

    

But now that I have added the property size

, I need to filter the array to select the correct image size.

See the following images to represent the data in my Realm file:

[

And I noticed examples of requests in the official Realm documentation :

// Query using a predicate string
RLMResults<Dog *> *tanDogs = [Dog objectsWhere:@"color = 'tan' AND name BEGINSWITH 'B'"];

// Query using an NSPredicate
NSPredicate *pred = [NSPredicate predicateWithFormat:@"color = %@ AND name BEGINSWITH %@",
                                                     @"tan", @"B"];
tanDogs = [Dog objectsWithPredicate:pred];

      

        
        
        
      

    

They look the same as what I am doing, so why am I seeing an exception?