diff --git a/src/lib/upnp/Discovery.cxx b/src/lib/upnp/Discovery.cxx
index 039f3a905..9a4b52712 100644
--- a/src/lib/upnp/Discovery.cxx
+++ b/src/lib/upnp/Discovery.cxx
@@ -42,12 +42,14 @@ UPnPDeviceDirectory::Downloader::Downloader(UPnPDeviceDirectory &_parent,
 	 expires(std::chrono::seconds(UpnpDiscovery_get_Expires(&disco))),
 	 request(*parent.curl, url.c_str(), *this)
 {
+	const std::lock_guard<Mutex> protect(parent.mutex);
 	parent.downloaders.push_back(*this);
 }
 
 void
 UPnPDeviceDirectory::Downloader::Destroy() noexcept
 {
+	const std::lock_guard<Mutex> protect(parent.mutex);
 	parent.downloaders.erase_and_dispose(parent.downloaders.iterator_to(*this),
 					     DeleteDisposer());
 }
@@ -265,6 +267,7 @@ UPnPDeviceDirectory::UPnPDeviceDirectory(EventLoop &event_loop,
 UPnPDeviceDirectory::~UPnPDeviceDirectory() noexcept
 {
 	BlockingCall(GetEventLoop(), [this](){
+			const std::lock_guard<Mutex> protect(mutex);
 			downloaders.clear_and_dispose(DeleteDisposer());
 		});
 }
diff --git a/src/lib/upnp/Discovery.hxx b/src/lib/upnp/Discovery.hxx
index 623b5d8dc..0750caf47 100644
--- a/src/lib/upnp/Discovery.hxx
+++ b/src/lib/upnp/Discovery.hxx
@@ -137,9 +137,15 @@ class UPnPDeviceDirectory final : UpnpCallback {
 
 	Mutex mutex;
 
+	/**
+	 * Protected by #mutex.
+	 */
 	boost::intrusive::list<Downloader,
 			       boost::intrusive::constant_time_size<false>> downloaders;
 
+	/**
+	 * Protected by #mutex.
+	 */
 	std::list<ContentDirectoryDescriptor> directories;
 
 	/**