diff --git a/imapclient/imapclient.py b/imapclient/imapclient.py
index 45ece113..39c5ca0a 100644
--- a/imapclient/imapclient.py
+++ b/imapclient/imapclient.py
@@ -830,7 +830,11 @@ def unselect_folder(self):
         """
         logger.debug("< UNSELECT")
         # IMAP4 class has no `unselect` method so we can't use `_command_and_check` there
-        _typ, data = self._imap._simple_command("UNSELECT")
+        typ, data = self._imap._simple_command("UNSELECT")
+        # Mirror imaplib.IMAP4.unselect: return the connection to the
+        # authenticated state so subsequent commands (e.g. ENABLE) are legal.
+        if typ == "OK":
+            self._imap.state = "AUTH"
         return data[0]
 
     def _process_select_response(self, resp):
diff --git a/tests/test_imapclient.py b/tests/test_imapclient.py
index 0819eb72..b6065b4d 100644
--- a/tests/test_imapclient.py
+++ b/tests/test_imapclient.py
@@ -337,6 +337,18 @@ def test_unselect(self):
         self.assertEqual(result, "Unselect completed.")
         self.client._imap._simple_command.assert_called_with("UNSELECT")
 
+    def test_unselect_resets_state_to_auth(self):
+        # A successful UNSELECT should return the connection to the
+        # authenticated state (mirroring imaplib.IMAP4.unselect) so that
+        # subsequent commands such as ENABLE are legal again. See #639.
+        self.client._cached_capabilities = [b"UNSELECT"]
+        self.client._imap.state = "SELECTED"
+        self.client._imap._simple_command.return_value = ("OK", ["Unselect completed."])
+
+        self.client.unselect_folder()
+
+        self.assertEqual(self.client._imap.state, "AUTH")
+
 
 class TestAppend(IMAPClientTest):
     def test_without_msg_time(self):