diff --git a/app/models/category.rb b/app/models/category.rb
index b7e71fc0834..66458783069 100644
--- a/app/models/category.rb
+++ b/app/models/category.rb
@@ -936,7 +936,7 @@ class Category < ActiveRecord::Base
   end
 
   def url
-    @@url_cache.defer_get_set(self.id) do
+    @@url_cache.defer_get_set(self.id.to_s) do
       "#{Discourse.base_path}/c/#{slug_path.join("/")}/#{self.id}"
     end
   end
diff --git a/lib/distributed_cache.rb b/lib/distributed_cache.rb
index bc710f26564..f39956ae1ff 100644
--- a/lib/distributed_cache.rb
+++ b/lib/distributed_cache.rb
@@ -14,6 +14,8 @@ class DistributedCache < MessageBus::DistributedCache
   end
 
   def defer_get_set(k, &block)
+    raise TypeError if !Rails.env.production? && !k.is_a?(String)
+
     return self[k] if hash.key? k
     value = block.call
     self.defer_set(k, value)