invalid model on an incremental QF_UFLIA instance about z3 HOT 1 CLOSED

[546] % z3release model_validate=true small.smt2 
sat
sat
sat
(error "line 7 column 10: an invalid model was generated")
(
  (define-fun a () Int
    0)
  (define-fun b ((x!0 Int)) Bool
    true)
)
[547] % cat small.smt2 
(declare-fun a () Int)
(check-sat)
(declare-fun b (Int) Bool)
(assert (b (+ a 1)))
(check-sat)
(assert (not (b 1)))
(check-sat)
(get-model)