1 files changed, 4 insertions, 0 deletions
diff --git a/version.py b/version.py
index af14cc13..c33bebae 100644
--- a/version.py
+++ b/version.py
@@ -166,6 +166,8 @@ class StrictVersion (Version):
     def _cmp (self, other):
         if isinstance(other, str):
             other = StrictVersion(other)
+        elif not isinstance(other, StrictVersion):
+            return NotImplemented
 
         if self.version != other.version:
             # numeric versions don't match
@@ -331,6 +333,8 @@ class LooseVersion (Version):
     def _cmp (self, other):
         if isinstance(other, str):
             other = LooseVersion(other)
+        elif not isinstance(other, LooseVersion):
+            return NotImplemented
 
         if self.version == other.version:
             return 0