Problem with var objects and adding values to them

[i_am_hal]

So I'm working on a little project, and I came upon a problem, I broke down the 
problem to illustrate what my problem is roughly. 
    
    
    type
        A = object of RootObj
            x: int
        
        B = object of A
            y: int
        
        X = object
            values: seq[A]
    
    var myX = X(values: @[])
    
    myX.values.add B(x:0, y:5)
    
    
    Run

This sort of code always gets the 'Invalid Object assignment' error. Even when 
I try to clarify that B is acceptable with 
    
    
    myX.values.add(A B(x:0, y:0))
    
    
    Run

I still get this error. What is going on? Is there a way for me to work around 
this? Thanks.