diff --git a/Lib/test/test_builtin.py b/Lib/test/test_builtin.py
--- a/Lib/test/test_builtin.py
+++ b/Lib/test/test_builtin.py
@@ -894,6 +894,12 @@
             fp.close()
         unlink(TESTFN)
 
+    def test_open_handles_NUL_chars(self):
+        self.write_testfile()
+        fn_with_NUL = TESTFN + '\0foobar'
+        self.assertRaises(TypeError, open, fn_with_NUL, 'r')
+        self.assertRaises(TypeError, open, bytes(fn_with_NUL, 'ascii'), 'r')
+
     def test_ord(self):
         self.assertEqual(ord(' '), 32)
         self.assertEqual(ord('A'), 65)
diff --git a/Modules/_io/fileio.c b/Modules/_io/fileio.c
--- a/Modules/_io/fileio.c
+++ b/Modules/_io/fileio.c
@@ -265,28 +265,11 @@
 #endif
     if (fd < 0)
     {
-        if (PyBytes_Check(nameobj) || PyByteArray_Check(nameobj)) {
-            Py_ssize_t namelen;
-            if (PyObject_AsCharBuffer(nameobj, &name, &namelen) < 0)
-                return -1;
+        PyObject *conv_name;
+        if (!PyUnicode_FSConverter(nameobj, &conv_name)) {
+            return -1;
         }
-        else {
-            PyObject *u = PyUnicode_FromObject(nameobj);
-
-            if (u == NULL)
-                return -1;
-
-            stringobj = PyUnicode_EncodeFSDefault(u);
-            Py_DECREF(u);
-            if (stringobj == NULL)
-                return -1;
-            if (!PyBytes_Check(stringobj)) {
-                PyErr_SetString(PyExc_TypeError,
-                                "encoder failed to return bytes");
-                goto error;
-            }
-            name = PyBytes_AS_STRING(stringobj);
-        }
+        name = PyBytes_AS_STRING(conv_name);
     }
 
     s = mode;