diff --git a/Python/bltinmodule.c b/Python/bltinmodule.c
index 069d790..10527c1 100644
--- a/Python/bltinmodule.c
+++ b/Python/bltinmodule.c
@@ -29,7 +29,7 @@
 int
 _Py_SetFileSystemEncoding(PyObject *s)
 {
-	PyObject *defenc;
+	PyObject *defenc, *codec;
 	if (!PyUnicode_Check(s)) {
 		PyErr_BadInternalCall();
 		return -1;
@@ -37,6 +37,10 @@
 	defenc = _PyUnicode_AsDefaultEncodedString(s, NULL);
 	if (!defenc)
 		return -1;
+	codec = _PyCodec_Lookup(PyBytes_AsString(defenc));
+	if (codec == NULL)
+		return -1;
+	Py_DECREF(codec);
 	if (!Py_HasFileSystemDefaultEncoding && Py_FileSystemDefaultEncoding)
 		/* A file system encoding was set at run-time */
 		free((char*)Py_FileSystemDefaultEncoding);