diff --git a/client.go b/client.go
index a71a4c8..172363b 100644
--- a/client.go
+++ b/client.go
@@ -61,6 +61,9 @@ func (m OptionMerchant) apply(c *Client) error {
 		return fmt.Errorf("InternalID %q already exists", m.InternalID)
 	}
 	c.merchants[m.InternalID] = m
+	if _, ok := c.merchants[m.MerchantID]; !ok {
+		c.merchants[m.MerchantID] = m
+	}
 	return nil
 }