From add721ef91ed533cf578ff7a604124e377329ae4 Mon Sep 17 00:00:00 2001
From: Emmanuel Odeke <emm.odeke@gmail.com>
Date: Mon, 24 Oct 2016 21:20:04 -0700
Subject: encoding/json: encode nil Marshaler as "null"

Fixes #16042.

Change-Id: I0a28aa004246b7b0ffaaab457e077ad9035363c2
Reviewed-on: https://go-review.googlesource.com/31932
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
---
 src/encoding/json/encode.go | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

(limited to 'src/encoding/json/encode.go')

diff --git a/src/encoding/json/encode.go b/src/encoding/json/encode.go
index 667df31ce3..98a9899502 100644
--- a/src/encoding/json/encode.go
+++ b/src/encoding/json/encode.go
@@ -443,7 +443,11 @@ func marshalerEncoder(e *encodeState, v reflect.Value, opts encOpts) {
 		e.WriteString("null")
 		return
 	}
-	m := v.Interface().(Marshaler)
+	m, ok := v.Interface().(Marshaler)
+	if !ok {
+		e.WriteString("null")
+		return
+	}
 	b, err := m.MarshalJSON()
 	if err == nil {
 		// copy JSON into buffer, checking validity.
-- 
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