Need to add additional object using NSCoder encodeObject:/decodeObject

So I have a project that uses the old-style encodeObject:/decodeObject NSCoder serialization. I need to add a new object to the serialization process but previously saved documents will crash as decodeObject tries to decode something that's not there (yet).

-(void) encodeWithCoder:(NSCoder *)aCoder {
    [super encodeWithCoder:aCoder];
    [aCoder encodeObject:existingArray];
    [aCoder encodeObject:anotherExistingArray];

    [aCoder encodeObject:myNewArray];
}

-(id) initWithCoder:(NSCoder *)aDecoder {
    self = [super initWithCoder:aDecoder];
    if( self != nil ) {
        NSArray *_existingArray = [aDecoder decodeObject];
        NSArray *_anotherExistingArray = [aDecoder decodeObject];

        // We crash here, this object doesn't exist in documents saved with only the first two arrays
        NSArray *_myNewArray = [aDecoder decodeObject]; 
    }
    return( self );
}

I don't think I can convert this to a keyed archive and maintain compatibility with previously saved archives (which is a requirement). Trying to catch the exception that's thrown doesn't seem to help either.

Any ideas?

UPDATE: Bah, why is it when you ask a question you almost immediately find the answer yourself?

This class was versioned correctly. Previous author had a bug in the version check. So for completeness:

+ (void) initialize {
    if( self == [MyClass class] ) {
        [self setVersio开发者_运维技巧n:2]; // was 1
    }
}

-(void) encodeWithCoder:(NSCoder *)aCoder {
    [super encodeWithCoder:aCoder];
    [aCoder encodeObject:existingArray];
    [aCoder encodeObject:anotherExistingArray];
    [aCoder encodeObject:myNewArray];
}

-(id) initWithCoder:(NSCoder *)aDecoder {
    self = [super initWithCoder:aDecoder];
    int version = [aDecoder versionForClassName:@"MyClass"];
    if( self != nil ) {
        NSArray *_existingArray = [aDecoder decodeObject];
        NSArray *_anotherExistingArray = [aDecoder decodeObject];

        if( version > 1 ) {
            NSArray *_myNewArray = [aDecoder decodeObject]; 
        }
    }
    return( self );
}

Continue to read old files (read-only) with serial archiver.
Load and save new files with keyed archiver, detect by:

[NSCoder allowsKeyedCoding]