proof unit-subsume-simplify Assertion violation at ../src/tactic/goal.cpp Line: 306

Hi,
For this formula:
```
(set-option :proof true)    
(assert (= true true)) 
(check-sat-using unit-subsume-simplify)     
```
Z3  throws out an assertion violation:
```
ASSERTION VIOLATION
File: ../src/tactic/goal.cpp
Line: 306
!m().proofs_enabled()
(C)ontinue, (A)bort, (S)top, (T)hrow exception, Invoke (G)DB
```

OS: Ubuntu 18.04
Commit: 65b2037
