diff --git a/src/lib/upnp/Discovery.cxx b/src/lib/upnp/Discovery.cxx
index 80157c707..f6a3ba122 100644
--- a/src/lib/upnp/Discovery.cxx
+++ b/src/lib/upnp/Discovery.cxx
@@ -212,7 +212,6 @@ UPnPDeviceDirectory::Invoke(Upnp_EventType et, void *evp)
 bool
 UPnPDeviceDirectory::ExpireDevices(Error &error)
 {
-	const ScopeLock protect(mutex);
 	const unsigned now = MonotonicClockS();
 	bool didsomething = false;
 
@@ -291,12 +290,11 @@ bool
 UPnPDeviceDirectory::GetDirectories(std::vector<ContentDirectoryService> &out,
 				    Error &error)
 {
-	// Has locking, do it before our own lock
+	const ScopeLock protect(mutex);
+
 	if (!ExpireDevices(error))
 		return false;
 
-	const ScopeLock protect(mutex);
-
 	for (auto dit = directories.begin();
 	     dit != directories.end(); dit++) {
 		for (const auto &service : dit->device.services) {
@@ -314,12 +312,11 @@ UPnPDeviceDirectory::GetServer(const char *friendly_name,
 			       ContentDirectoryService &server,
 			       Error &error)
 {
-	// Has locking, do it before our own lock
+	const ScopeLock protect(mutex);
+
 	if (!ExpireDevices(error))
 		return false;
 
-	const ScopeLock protect(mutex);
-
 	for (const auto &i : directories) {
 		const auto &device = i.device;
 
diff --git a/src/lib/upnp/Discovery.hxx b/src/lib/upnp/Discovery.hxx
index 970fb306e..1cf82b77e 100644
--- a/src/lib/upnp/Discovery.hxx
+++ b/src/lib/upnp/Discovery.hxx
@@ -138,6 +138,8 @@ private:
 	 * Look at the devices and get rid of those which have not
 	 * been seen for too long. We do this when listing the top
 	 * directory.
+	 *
+	 * Caller must lock #mutex.
 	 */
 	bool ExpireDevices(Error &error);