diff --git a/src/encoding/json/encode.go b/src/encoding/json/encode.go
index ea5eca51ef..483b9d8f2d 100644
--- a/src/encoding/json/encode.go
+++ b/src/encoding/json/encode.go
@@ -236,6 +236,8 @@ func (e *UnsupportedTypeError) Error() string {
 	return "json: unsupported type: " + e.Type.String()
 }
 
+// An UnsupportedValueError is returned by Marshal when attempting
+// to encode an unsupported value.
 type UnsupportedValueError struct {
 	Value reflect.Value
 	Str   string