Easily reproduced on master, thanks

(lldb) run encode_crash.py
Process 14743 launched: '/Users/anthonyshaw/repo/cpython/python.exe' (x86_64)
Objects/unicodeobject.c:448: _PyUnicode_CheckConsistency: Assertion "((((((PyObject*)(op))->ob_type))->tp_flags & ((1UL << 28))) != 0)" failed
Enable tracemalloc to get the memory block allocation traceback

object  : Process 14743 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=EXC_I386_GPFLT)
    frame #0: 0x00000001000b5c15 python.exe`PyObject_Repr(v=0x0000000101376f90) at object.c:535:11
   532 	       infinitely. */
   533 	    if (Py_EnterRecursiveCall(" while getting the repr of an object"))
   534 	        return NULL;
-> 535 	    res = (*v->ob_type->tp_repr)(v);
   536 	    Py_LeaveRecursiveCall();
   537 	    if (res == NULL)
   538 	        return NULL;