diff --git a/client/src/pages/guide/english/rest-api/index.md b/client/src/pages/guide/english/rest-api/index.md
index 94ee3974b5..ecffb53676 100644
--- a/client/src/pages/guide/english/rest-api/index.md
+++ b/client/src/pages/guide/english/rest-api/index.md
@@ -103,6 +103,7 @@ REST was developed to provide a uniform interface for
   The dangerous **5xx** resources!
 
   - **500** Internal Server Error
+  - **501** Not implemented. Server lacks the ability to fulfil the request
   - **504** Gateway Timeout. Server didn't receive timely response
 
   Less known **4xx** suggests that you are passing wrong parameter. Can also pass information that is wrong. E.g.