diff --git a/railties/lib/rails_generator/lookup.rb b/railties/lib/rails_generator/lookup.rb
index 598cb79e73..d88a1948a4 100644
--- a/railties/lib/rails_generator/lookup.rb
+++ b/railties/lib/rails_generator/lookup.rb
@@ -139,7 +139,7 @@ module Rails
         private
           # Lookup and cache every generator from the source list.
           def cache
-            @cache ||= sources.inject([]) { |cache, source| cache + source.map }
+            @cache ||= sources.inject([]) { |cache, source| cache + source.to_a }
           end
 
           # Clear the cache whenever the source list changes.