psmt assertion violation at ../src/smt/smt_context.h Line: 277

Hi,
For this formula:
```
(declare-fun a (Int) Bool)
(declare-fun b (Int) Bool)
(assert (distinct a b))
(check-sat-using psmt)

```
Z3  throws out an assertion violation:
```
ASSERTION VIOLATION
File: ../src/smt/smt_context.h
Line: 277
e_internalized(n)
(C)ontinue, (A)bort, (S)top, (T)hrow exception, Invoke (G)DB

```

OS: Ubuntu 18.04
Commit: 2882a67
