diff --git a/src/pkg/encoding/json/encode.go b/src/pkg/encoding/json/encode.go
index 5425a3a90a1..edbafcf65f1 100644
--- a/src/pkg/encoding/json/encode.go
+++ b/src/pkg/encoding/json/encode.go
@@ -43,7 +43,8 @@ import (
 // to keep some browsers from misinterpreting JSON output as HTML.
 //
 // Array and slice values encode as JSON arrays, except that
-// []byte encodes as a base64-encoded string.
+// []byte encodes as a base64-encoded string, and a nil slice
+// encodes as the null JSON object.
 //
 // Struct values encode as JSON objects. Each exported struct field
 // becomes a member of the object unless