1 files changed, 6 insertions, 2 deletions
diff --git a/activesupport/lib/active_support/core_ext/enumerable.rb b/activesupport/lib/active_support/core_ext/enumerable.rb
index 02d5a7080f..03efe6a19a 100644
--- a/activesupport/lib/active_support/core_ext/enumerable.rb
+++ b/activesupport/lib/active_support/core_ext/enumerable.rb
@@ -65,11 +65,15 @@ class Range #:nodoc:
   # Optimize range sum to use arithmetic progression if a block is not given and
   # we have a range of numeric values.
   def sum(identity = 0)
-    if block_given? || !(first.instance_of?(Integer) && last.instance_of?(Integer))
+    if block_given? || !(first.is_a?(Integer) && last.is_a?(Integer))
       super
     else
       actual_last = exclude_end? ? (last - 1) : last
-      (actual_last - first + 1) * (actual_last + first) / 2
+      if actual_last >= first
+        (actual_last - first + 1) * (actual_last + first) / 2
+      else
+        identity
+      end
     end
   end
 end