weird inheritance rule restriction

[jangko]

type
  Curve1[T, X] = object of RootObj
    a: T
    b: X
    
  Curve2[T] = Curve1[T, int]
  
  Curve3[T] = object of Curve2[T]

the above code trigger unexpected error:
curve.nim(8, 22) Error: inheritance only works with non-final objects

is this a bug or not? why inheritance not work with Curve2