diff --git a/.rubocop.yml b/.rubocop.yml
index 8e00e2dc..db25cbf5 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -45,6 +45,13 @@ Style/ClassAndModuleChildren:
 Style/Documentation:
   Enabled: false
 
+Style/MissingElse:
+  Enabled: true
+  EnforcedStyle: case
+
+Style/EmptyElse:
+  EnforcedStyle: empty
+
 Style/MultilineOperationIndentation:
   EnforcedStyle: indented
 
diff --git a/lib/active_model/serializer/include_tree.rb b/lib/active_model/serializer/include_tree.rb
index d0f0f731..28bc2175 100644
--- a/lib/active_model/serializer/include_tree.rb
+++ b/lib/active_model/serializer/include_tree.rb
@@ -102,6 +102,8 @@ module ActiveModel
           self.class.new(@hash[:*])
         when @hash.key?(:**)
           self.class.new(:** => {})
+        else
+          nil
         end
       end
     end