diff --git a/optional_unmarshaljson.go b/optional_unmarshaljson.go
index f17e80e..164fd4c 100644
--- a/optional_unmarshaljson.go
+++ b/optional_unmarshaljson.go
@@ -11,10 +11,10 @@ var _ json.Unmarshaler = new(Optional[string])
 // UnmarshalJSON makes it so json.Unmarshaler is implemented.
 func (receiver *Optional[T]) UnmarshalJSON(data []byte) error {
 	switch interface{}(receiver.value).(type) {
-	case bool, string:
+	case bool, string, json.Unmarshaler:
 		// these are OK.
 	default:
-		return fmt.Errorf("cannot unmarshal into something of type %T from JSON because parameterized type is ‘%T’ rather than ‘bool’ or ‘string’", receiver, receiver.value)
+		return fmt.Errorf("cannot unmarshal into something of type %T from JSON because parameterized type is ‘%T’ rather than ‘bool’, ‘string’, or ‘json.Unmarshaler’", receiver, receiver.value)
 	}
 
 	{