diff --git a/lib/statsample.rb b/lib/statsample.rb
index ce5cc8b..1352a54 100644
--- a/lib/statsample.rb
+++ b/lib/statsample.rb
@@ -54,8 +54,10 @@ def include_aliasing(m, suffix = 'ruby')
 end
 
 class Array
-  def sum
-    inject(:+)
+  unless method_defined?(:sum)
+    def sum
+      inject(:+)
+    end
   end
 
   def mean