diff --git a/lib/graphql/execution/finalize.rb b/lib/graphql/execution/finalize.rb
index 34ad59432e..1cafa579d2 100644
--- a/lib/graphql/execution/finalize.rb
+++ b/lib/graphql/execution/finalize.rb
@@ -28,7 +28,7 @@ def initialize(query, data, runner)
           targets = [data]
           while (part = err_path.shift)
             targets.map! { |t| t[part] }
-            targets.flatten!
+            targets = targets.flatten
           end
 
           targets.each_with_index do |target, idx|