(proof=true) invalid model issue

[zhendongsu]

Commit: 9061ca5
OS: Ubuntu 18.04

[534] % z3release model_validate=true small.smt2
sat
(
  (define-fun e () b
    (d 2))
)
[535] % z3release model_validate=true proof=true small.smt2
sat
(error "line 4 column 10: an invalid model was generated")
(
  (define-fun e () b
    c)
)
[536] % cat small.smt2
(declare-datatypes () ((b c (d (a Int)))))
(declare-const e b)
(assert ((_ is d) ((_ update-field a) e 1)))
(check-sat)
(get-model)