diff --git a/twain/twain/huagaods.cpp b/twain/twain/huagaods.cpp
index dd04e39..bedc467 100644
--- a/twain/twain/huagaods.cpp
+++ b/twain/twain/huagaods.cpp
@@ -1723,6 +1723,9 @@ Result huagao_ds::capCommon(const Identity&, Msg msg, Capability& data) {
 }
 Twpp::Result huagao_ds::showTwainUI(Twpp::UserInterface& data, bool bUiOnly)
 {
+	if (!scanner_->is_online())
+		return { ReturnCode::Failure,  huagao_ds::condition_code_from_hg_error(SCANNER_ERR_DEVICE_NOT_FOUND) };
+
 	show_setting_ = true;
 	bUiOnly_ = bUiOnly;
 	// display user UI ... (setting UI, can we show my own main window here ?)