diff --git a/engines/engine.cpp b/engines/engine.cpp
index f4dbbfeed6b..10fca8cfa88 100644
--- a/engines/engine.cpp
+++ b/engines/engine.cpp
@@ -814,6 +814,23 @@ bool Engine::warnUserAboutUnsupportedGame(Common::String msg) {
 	return true;
 }
 
+void Engine::errorAddingAddOnWithoutBaseGame(Common::String addOnName, Common::String gameId) {
+	Common::TextToSpeechManager *ttsMan = g_system->getTextToSpeechManager();
+	if (ttsMan != nullptr) {
+		ttsMan->pushState();
+		g_gui.initTextToSpeech();
+	}
+
+	Common::U32String messageFormat = _("The game \"%s\" you are trying to add is an add-on for \"%s\" that cannot be run independently."
+		" Please copy the add-on contents into a subdirectory of the base game, and start the base game itself.");
+	Common::U32String message = Common::U32String::format(messageFormat, addOnName.c_str(), gameId.c_str());
+
+	GUI::MessageDialog(message).runModal();
+
+	if (ttsMan != nullptr)
+		ttsMan->popState();
+}
+
 void Engine::errorUnsupportedGame(Common::String extraMsg) {
 	Common::TextToSpeechManager *ttsMan = g_system->getTextToSpeechManager();
 	if (ttsMan != nullptr) {
diff --git a/engines/engine.h b/engines/engine.h
index 90da7dcecb9..d60c11d434d 100644
--- a/engines/engine.h
+++ b/engines/engine.h
@@ -591,6 +591,16 @@ public:
 	 */
 	static bool warnUserAboutUnsupportedGame(Common::String msg = Common::String());
 
+	/**
+	/**
+	 * Display an error message to the user that the game is an add-on than cannot be
+	 * run independently.
+	 *
+	 * @param addOnName The name of the add-on.
+	 * @param gameId    The ID of the base game that this add-on requires.
+	 */
+	static void errorAddingAddOnWithoutBaseGame(Common::String addOnName, Common::String gameId);
+
 	/**
 	 * Display an error message to the user that the game is not supported.
 	 *
diff --git a/gui/launcher.cpp b/gui/launcher.cpp
index 6cbf4a91f15..de514473761 100644
--- a/gui/launcher.cpp
+++ b/gui/launcher.cpp
@@ -730,6 +730,11 @@ bool LauncherDialog::doGameDetection(const Common::Path &path) {
 
 	if (0 <= idx && idx < (int)candidates.size()) {
 		const DetectedGame &result = candidates[idx];
+		if (result.isAddOn) {
+			Engine::errorAddingAddOnWithoutBaseGame(result.description, result.gameId);
+			return true;
+		}
+
 		Common::String domain = EngineMan.createTargetForGame(result);
 
 		// Display edit dialog for the new entry
diff --git a/gui/massadd.cpp b/gui/massadd.cpp
index 1eddf6d9879..9aca458f1f5 100644
--- a/gui/massadd.cpp
+++ b/gui/massadd.cpp
@@ -199,7 +199,7 @@ void MassAddDialog::handleTickle() {
 		}
 
 		// Run the detector on the dir
-		DetectionResults detectionResults = EngineMan.detectGames(files, (ADGF_WARNING | ADGF_UNSUPPORTED), true);
+		DetectionResults detectionResults = EngineMan.detectGames(files, (ADGF_WARNING | ADGF_UNSUPPORTED | ADGF_ADDON), true);
 
 		if (detectionResults.foundUnknownGames()) {
 			Common::U32String report = detectionResults.generateUnknownGameReport(false, 80);