diff --git a/django/db/models/query.py b/django/db/models/query.py
index 6b341ba9ab..12731caa94 100644
--- a/django/db/models/query.py
+++ b/django/db/models/query.py
@@ -292,6 +292,8 @@ class QuerySet(object):
         Updates all elements in the current QuerySet, setting all the given
         fields to the appropriate values.
         """
+        assert self.query.can_filter(), \
+                "Cannot update a query once a slice has been taken."
         query = self.query.clone(sql.UpdateQuery)
         query.add_update_values(kwargs)
         query.execute_sql(None)
@@ -306,6 +308,8 @@ class QuerySet(object):
         code (it requires too much poking around at model internals to be
         useful at that level).
         """
+        assert self.query.can_filter(), \
+                "Cannot update a query once a slice has been taken."
         query = self.query.clone(sql.UpdateQuery)
         query.add_update_fields(values)
         query.execute_sql(None)
diff --git a/tests/modeltests/update/models.py b/tests/modeltests/update/models.py
index 3b0f83389f..8a35b61a7c 100644
--- a/tests/modeltests/update/models.py
+++ b/tests/modeltests/update/models.py
@@ -63,5 +63,12 @@ a manager method.
 >>> DataPoint.objects.values('value').distinct()
 [{'value': u'thing'}]
 
+We do not support update on already sliced query sets.
+
+>>> DataPoint.objects.all()[:2].update(another_value='another thing')
+Traceback (most recent call last):
+    ...
+AssertionError: Cannot update a query once a slice has been taken.
+
 """
 }