diff --git a/Objects/fileobject.c b/Objects/fileobject.c
index 90c9687..1880187 100644
--- a/Objects/fileobject.c
+++ b/Objects/fileobject.c
@@ -354,6 +354,8 @@
 {
 	PyFileObject *file = (PyFileObject*)f;
 	PyObject *str = PyString_FromString(enc);
+
+	assert(PyFile_Check(f));
 	if (!str)
 		return 0;
 	Py_DECREF(file->f_encoding);