From fff5b4cae4628d6c2e47d859d920f060bca118dd Mon Sep 17 00:00:00 2001
From: Zach Gershman <zachgersh@users.noreply.github.com>
Date: Tue, 3 Sep 2019 08:32:39 -0700
Subject: [PATCH] docs need type assertion on proto.Message (#1026)

* docs need type assertion on proto.Message

* Update docs/_docs/customizingyourgateway.md

Co-Authored-By: Johan Brandhorst <johan.brandhorst@gmail.com>
---
 docs/_docs/customizingyourgateway.md | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/docs/_docs/customizingyourgateway.md b/docs/_docs/customizingyourgateway.md
index 73286d9f8c9..a22ce80594c 100644
--- a/docs/_docs/customizingyourgateway.md
+++ b/docs/_docs/customizingyourgateway.md
@@ -104,8 +104,13 @@ Or you might want to mutate the response messages to be returned.
 1. Write a filter function.
    ```go
    func myFilter(ctx context.Context, w http.ResponseWriter, resp proto.Message) error {
-   	w.Header().Set("X-My-Tracking-Token", resp.Token)
-   	resp.Token = ""
+        t, ok := resp.(*externalpb.Tokenizer)
+
+	if ok {
+	  w.Header().Set("X-My-Tracking-Token", t.Token)
+	  t.Token = ""
+	}
+
    	return nil
    }
    ```