diff --git a/Misc/NEWS.d/next/Core and Builtins/2017-09-13-13-03-52.bpo-31418.rS-FlC.rst b/Misc/NEWS.d/next/Core and Builtins/2017-09-13-13-03-52.bpo-31418.rS-FlC.rst
new file mode 100644
index 00000000000..6d6cbd81142
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2017-09-13-13-03-52.bpo-31418.rS-FlC.rst	
@@ -0,0 +1,2 @@
+Fix an assertion failure in `PyErr_WriteUnraisable()` in case of an
+exception with a bad ``__module__`` attribute. Patch by Oren Milman.
diff --git a/Python/errors.c b/Python/errors.c
index 3cb6d5306f8..5709fddb584 100644
--- a/Python/errors.c
+++ b/Python/errors.c
@@ -961,7 +961,7 @@ PyErr_WriteUnraisable(PyObject *obj)
     }
 
     moduleName = _PyObject_GetAttrId(t, &PyId___module__);
-    if (moduleName == NULL) {
+    if (moduleName == NULL || !PyUnicode_Check(moduleName)) {
         PyErr_Clear();
         if (PyFile_WriteString("<unknown>", f) < 0)
             goto done;