proof qffp Assertion violation at ../src/tactic/goal.cpp Line: 291

Hi,
For this formula:
```
(set-option :proof true)
(assert (forall ((a (_ FloatingPoint 11 53))) (fp.isNaN a)))
(check-sat-using qffp)
```
Z3  throws out an assertion violation:
```
ASSERTION VIOLATION
File: ../src/tactic/goal.cpp
Line: 291
f == m().get_fact(pr)
```

OS: Ubuntu 18.04
Commit: 337c07a
