diff --git a/solutions/system_design/sales_rank/README.md b/solutions/system_design/sales_rank/README.md
index 71ad1c7d..892a9d3d 100644
--- a/solutions/system_design/sales_rank/README.md
+++ b/solutions/system_design/sales_rank/README.md
@@ -126,7 +126,7 @@ class SalesRanker(MRJob):
         if self.within_past_week(timestamp):
             yield (category_id, product_id), quantity
 
-    def reducer(self, key, value):
+    def reducer(self, key, values):
         """Sum values for each key.
 
         (category1, product1), 2