diff --git a/docs/service.go b/docs/service.go
index c27f9b1c..3006e8ab 100644
--- a/docs/service.go
+++ b/docs/service.go
@@ -37,6 +37,9 @@ func (s *Service) ServiceConfig() *registrytypes.ServiceConfig {
 // It can be used to verify the validity of a client's credentials.
 func (s *Service) Auth(authConfig *types.AuthConfig, userAgent string) (status, token string, err error) {
 	serverAddress := authConfig.ServerAddress
+	if serverAddress == "" {
+		serverAddress = IndexServer
+	}
 	if !strings.HasPrefix(serverAddress, "https://") && !strings.HasPrefix(serverAddress, "http://") {
 		serverAddress = "https://" + serverAddress
 	}