diff -r a415b03809f3 Lib/collections/__init__.py
--- a/Lib/collections/__init__.py	Wed Oct 29 10:57:42 2014 +0100
+++ b/Lib/collections/__init__.py	Wed Oct 29 08:53:01 2014 -0400
@@ -735,6 +735,8 @@
         Counter({'b': 4, 'c': 2, 'a': 1})
 
         '''
+        if not isinstance(other, Counter):
+            return NotImplemented
         for elem, count in other.items():
             self[elem] += count
         return self._keep_positive()
@@ -748,6 +750,8 @@
         Counter({'b': 2, 'a': 1})
 
         '''
+        if not isinstance(other, Counter):
+            return NotImplemented
         for elem, count in other.items():
             self[elem] -= count
         return self._keep_positive()
@@ -761,6 +765,8 @@
         Counter({'b': 3, 'c': 2, 'a': 1})
 
         '''
+        if not isinstance(other, Counter):
+            return NotImplemented
         for elem, other_count in other.items():
             count = self[elem]
             if other_count > count:
@@ -776,6 +782,8 @@
         Counter({'b': 1})
 
         '''
+        if not isinstance(other, Counter):
+            return NotImplemented
         for elem, count in self.items():
             other_count = other[elem]
             if other_count < count: