This was the cause:

• We checked that passed arg is not adict here:

  cpython/Python/crossinterp.c

  Lines 1479 to 1482 in6ee542d

  	if (!PyDict_CheckExact(nsobj)) {
  	PyErr_SetString(PyExc_TypeError,"expected a dict");
  	returnNULL;
  	}

• Later we had an assertion about!PyErr_Occurred()

So, I went with the early validation.

• Issue:_interpreters.exec with invalid parameters segfaults #126654