diff --git a/src/libopensc/card-coolkey.c b/src/libopensc/card-coolkey.c
index d5193364..c44febe6 100644
--- a/src/libopensc/card-coolkey.c
+++ b/src/libopensc/card-coolkey.c
@@ -1360,6 +1360,7 @@ static int coolkey_get_serial_nr_from_CUID(sc_card_t* card, sc_serial_number_t*
 
 	SC_FUNC_CALLED(card->ctx, SC_LOG_DEBUG_NORMAL);
 	memcpy(serial->value, &priv->cuid, sizeof(priv->cuid));
+	serial->len = sizeof(priv->cuid);
 	SC_FUNC_RETURN(card->ctx, SC_LOG_DEBUG_NORMAL, SC_SUCCESS);
 }