Assertion violation at ../src/tactic/fd_solver/smtfd_solver.cpp Line: 1928

Hi,
For this formula,
```
(assert (forall ((a (_ BitVec 1))) (forall ((b (_ BitVec 1))) (= b #b0))))
(check-sat-using smtfd)
```
z3 throws out an assertion violation:
```
ASSERTION VIOLATION
File: ../src/tactic/fd_solver/smtfd_solver.cpp
Line: 1928
!m_axioms.contains(fml)
(C)ontinue, (A)bort, (S)top, (T)hrow exception, Invoke (G)DB
a
```

OS: Ubuntu 18.04
Revision: 28c827f