diff --git a/pymispgalaxies/__init__.py b/pymispgalaxies/__init__.py
index 795d66f..139bff8 100644
--- a/pymispgalaxies/__init__.py
+++ b/pymispgalaxies/__init__.py
@@ -1,4 +1,4 @@
 #!/usr/bin/env python
 # -*- coding: utf-8 -*-
 
-from .api import Galaxies, Clusters, EncodeGalaxies
+from .api import Galaxies, Clusters, EncodeGalaxies, UnableToRevertMachinetag
diff --git a/pymispgalaxies/api.py b/pymispgalaxies/api.py
index 179ddb3..6ab4e17 100644
--- a/pymispgalaxies/api.py
+++ b/pymispgalaxies/api.py
@@ -244,8 +244,8 @@ class Clusters(collections.Mapping):
         return [cluster.machinetags() for cluster in self.values()]
 
     def revert_machinetag(self, machinetag):
-        _, cluster_type, cluster_value = re.findall('^([^:]*):([^=]*)="([^"]*)"$', machinetag)[0]
         try:
+            _, cluster_type, cluster_value = re.findall('^([^:]*):([^=]*)="([^"]*)"$', machinetag)[0]
             cluster = self.get(cluster_type)
             value = cluster[cluster_value]
             return cluster, value
diff --git a/tests/tests.py b/tests/tests.py
index 5e53b9b..9a88bde 100644
--- a/tests/tests.py
+++ b/tests/tests.py
@@ -2,7 +2,7 @@
 # -*- coding: utf-8 -*-
 
 import unittest
-from pymispgalaxies import Galaxies, Clusters
+from pymispgalaxies import Galaxies, Clusters, UnableToRevertMachinetag
 from glob import glob
 import os
 import json
@@ -59,6 +59,8 @@ class TestPyMISPGalaxies(unittest.TestCase):
 
     def test_revert_machinetag(self):
         self.assertEqual(len(self.clusters.revert_machinetag('misp-galaxy:tool="Babar"')), 2)
+        with self.assertRaises(UnableToRevertMachinetag):
+            self.clusters.revert_machinetag('blah')
 
     def test_len(self):
         self.assertIsNot(len(self.clusters), 0)