Invalid model on string formula

[merlinsun]

Hi,
For this instance, z3 56aa426 gives an invalid model.

$ z3 model_validate=true delta.smt2
sat
(error "line 5 column 10: an invalid model was generated")
$ cat delta.smt2
(declare-fun v () Int)
(declare-fun r () String)
(assert (distinct r (str.substr r 0 (div 0 (* v v v v v))) "fu" ""))
(assert (exists ((e Bool)) e))
(check-sat)