diff --git a/src/encoding/json/encode.go b/src/encoding/json/encode.go
index e59a0b3bbf..4db9f35e69 100644
--- a/src/encoding/json/encode.go
+++ b/src/encoding/json/encode.go
@@ -113,8 +113,8 @@ import (
 // a JSON tag of "-".
 //
 // Map values encode as JSON objects.
-// The map's key type must be string; the object keys are used directly
-// as map keys.
+// The map's key type must be string; the map keys are used as JSON object
+// keys, subject to the UTF-8 coercion described for string values above.
 //
 // Pointer values encode as the value pointed to.
 // A nil pointer encodes as the null JSON object.