diff --git a/libs/pbd/pbd/abstract_ui.cc b/libs/pbd/pbd/abstract_ui.cc
index ae3f9d49c9..4650fafcbd 100644
--- a/libs/pbd/pbd/abstract_ui.cc
+++ b/libs/pbd/pbd/abstract_ui.cc
@@ -431,13 +431,13 @@ AbstractUI<RequestObject>::send_request (RequestObject *req)
 	}
 }
 
-template<typename RequestObject> void
+template<typename RequestObject> bool
 AbstractUI<RequestObject>::call_slot (InvalidationRecord* invalidation, const boost::function<void()>& f)
 {
 	if (caller_is_self()) {
 		DEBUG_TRACE (PBD::DEBUG::AbstractUI, string_compose ("%1/%2 direct dispatch of call slot via functor @ %3, invalidation %4\n", event_loop_name(), pthread_name(), &f, invalidation));
 		f ();
-		return;
+		return true;
 	}
 
 	/* object destruction may race with realtime signal emission.
@@ -453,7 +453,7 @@ AbstractUI<RequestObject>::call_slot (InvalidationRecord* invalidation, const bo
 	if (invalidation) {
 		if (!invalidation->valid()) {
 			DEBUG_TRACE (PBD::DEBUG::AbstractUI, string_compose ("%1/%2 ignoring call-slot using functor @ %3, dead invalidation %4\n", event_loop_name(), pthread_name(), &f, invalidation));
-			return;
+			return true;
 		}
 		invalidation->ref ();
 		invalidation->event_loop = this;
@@ -465,7 +465,10 @@ AbstractUI<RequestObject>::call_slot (InvalidationRecord* invalidation, const bo
 		if (invalidation) {
 			invalidation->unref ();
 		}
-		return;
+		/* event is lost, this can be critical in some cases, so
+		 * inform the caller. See also Session::process_rtop
+		 */
+		return false;
 	}
 
 	DEBUG_TRACE (PBD::DEBUG::AbstractUI, string_compose ("%1/%2 queue call-slot using functor @ %3, invalidation %4\n", event_loop_name(), pthread_name(), &f, invalidation));
@@ -485,6 +488,7 @@ AbstractUI<RequestObject>::call_slot (InvalidationRecord* invalidation, const bo
 	req->invalidation = invalidation;
 
 	send_request (req);
+	return true;
 }
 
 template<typename RequestObject> void*
diff --git a/libs/pbd/pbd/abstract_ui.h b/libs/pbd/pbd/abstract_ui.h
index b63b9b7c6b..7c0083459a 100644
--- a/libs/pbd/pbd/abstract_ui.h
+++ b/libs/pbd/pbd/abstract_ui.h
@@ -60,7 +60,7 @@ public:
 	virtual ~AbstractUI();
 
 	void register_thread (pthread_t, std::string, uint32_t num_requests);
-	void call_slot (EventLoop::InvalidationRecord*, const boost::function<void()>&);
+	bool call_slot (EventLoop::InvalidationRecord*, const boost::function<void()>&);
 	Glib::Threads::Mutex& slot_invalidation_mutex() { return request_buffer_map_lock; }
 
 	Glib::Threads::Mutex request_buffer_map_lock;
@@ -98,7 +98,7 @@ protected:
 
 	RequestObject* get_request (RequestType);
 	void handle_ui_requests ();
-	void send_request (RequestObject *);
+	void send_request (RequestObject*);
 
 	virtual void do_request (RequestObject *) = 0;
 	PBD::ScopedConnection new_thread_connection;
diff --git a/libs/pbd/pbd/event_loop.h b/libs/pbd/pbd/event_loop.h
index 44d2bca475..6a1504abc4 100644
--- a/libs/pbd/pbd/event_loop.h
+++ b/libs/pbd/pbd/event_loop.h
@@ -88,7 +88,7 @@ public:
 		}
 	};
 
-	virtual void call_slot (InvalidationRecord*, const boost::function<void()>&) = 0;
+	virtual bool call_slot (InvalidationRecord*, const boost::function<void()>&) = 0;
 	virtual Glib::Threads::Mutex& slot_invalidation_mutex() = 0;
 
 	std::string event_loop_name() const { return _name; }
diff --git a/luasession/luasession.cc b/luasession/luasession.cc
index 50e1f34cb3..d4ad68d140 100644
--- a/luasession/luasession.cc
+++ b/luasession/luasession.cc
@@ -109,7 +109,7 @@ public:
 		run_loop_thread = Glib::Threads::Thread::self ();
 	}
 
-	void call_slot (InvalidationRecord* ir, const boost::function<void()>& f)
+	bool call_slot (InvalidationRecord* ir, const boost::function<void()>& f)
 	{
 		if (Glib::Threads::Thread::self () == run_loop_thread) {
 			cout << string_compose ("%1/%2 direct dispatch of call slot via functor @ %3, invalidation %4\n", event_loop_name (), pthread_name (), &f, ir);
@@ -119,6 +119,7 @@ public:
 			assert (!ir);
 			f (); // XXX TODO, queue and process during run ()
 		}
+		return true;
 	}
 
 	void run ()
diff --git a/session_utils/common.cc b/session_utils/common.cc
index 1345cee063..6662406352 100644
--- a/session_utils/common.cc
+++ b/session_utils/common.cc
@@ -96,10 +96,11 @@ class MyEventLoop : public sigc::trackable, public EventLoop
 			run_loop_thread = Glib::Threads::Thread::self();
 		}
 
-		void call_slot (InvalidationRecord*, const boost::function<void()>& f) {
+		bool call_slot (InvalidationRecord*, const boost::function<void()>& f) {
 			if (Glib::Threads::Thread::self() == run_loop_thread) {
 				f ();
 			}
+			return true;
 		}
 
 		Glib::Threads::Mutex& slot_invalidation_mutex() { return request_buffer_map_lock; }