diff --git a/libs/gtkmm2ext/gtk_ui.cc b/libs/gtkmm2ext/gtk_ui.cc
index f6acfab981..af9d2bd198 100644
--- a/libs/gtkmm2ext/gtk_ui.cc
+++ b/libs/gtkmm2ext/gtk_ui.cc
@@ -456,23 +456,6 @@ UI::idle_add (int (*func)(void *), void *arg)
 
 /* END abstract_ui interfaces */
 
-/** Create a PBD::EventLoop::InvalidationRecord and attach a callback
- *  to a given sigc::trackable so that PBD::EventLoop::invalidate_request
- *  is called when that trackable is destroyed.
- */
-PBD::EventLoop::InvalidationRecord*
-__invalidator (sigc::trackable& trackable, const char* file, int line)
-{
-        PBD::EventLoop::InvalidationRecord* ir = new PBD::EventLoop::InvalidationRecord;
-
-        ir->file = file;
-        ir->line = line;
-
-        trackable.add_destroy_notify_callback (ir, PBD::EventLoop::invalidate_request);
-
-        return ir;
-}
-
 void
 UI::do_request (UIRequest* req)
 {
diff --git a/libs/gtkmm2ext/gtkmm2ext/gui_thread.h b/libs/gtkmm2ext/gtkmm2ext/gui_thread.h
index 66c381c073..63786ed90f 100644
--- a/libs/gtkmm2ext/gtkmm2ext/gui_thread.h
+++ b/libs/gtkmm2ext/gtkmm2ext/gui_thread.h
@@ -35,7 +35,6 @@ namespace sigc {
 #define gui_context() Gtkmm2ext::UI::instance() /* a UICallback-derived object that specifies the event loop for GUI signal handling */
 #define ui_bind(f, ...) boost::protect (boost::bind (f, __VA_ARGS__))
 
-LIBGTKMM2EXT_API extern PBD::EventLoop::InvalidationRecord* __invalidator (sigc::trackable& trackable, const char*, int);
-#define invalidator(x) __invalidator ((x), __FILE__, __LINE__)
+#define invalidator(x) PBD::EventLoop::__invalidator ((x), __FILE__, __LINE__)
 
 #endif /* __ardour_gtk_gui_thread_h__ */
diff --git a/libs/pbd/event_loop.cc b/libs/pbd/event_loop.cc
index 97a6dc4ee0..a0dae62fef 100644
--- a/libs/pbd/event_loop.cc
+++ b/libs/pbd/event_loop.cc
@@ -109,6 +109,24 @@ EventLoop::invalidate_request (void* data)
 	return 0;
 }
 
+/** Create a PBD::EventLoop::InvalidationRecord and attach a callback
+ *  to a given sigc::trackable so that PBD::EventLoop::invalidate_request
+ *  is called when that trackable is destroyed.
+ */
+EventLoop::InvalidationRecord*
+EventLoop::__invalidator (sigc::trackable& trackable, const char* file, int line)
+{
+        PBD::EventLoop::InvalidationRecord* ir = new PBD::EventLoop::InvalidationRecord;
+
+        ir->file = file;
+        ir->line = line;
+
+        trackable.add_destroy_notify_callback (ir, PBD::EventLoop::invalidate_request);
+
+        return ir;
+}
+
+
 vector<EventLoop::ThreadBufferMapping>
 EventLoop::get_request_buffers_for_target_thread (const std::string& target_thread)
 {
@@ -197,3 +215,4 @@ EventLoop::remove_request_buffer_from_map (pthread_t pth)
 		}
 	}
 }
+
diff --git a/libs/pbd/pbd/event_loop.h b/libs/pbd/pbd/event_loop.h
index 5f24bbeeba..48df8620df 100644
--- a/libs/pbd/pbd/event_loop.h
+++ b/libs/pbd/pbd/event_loop.h
@@ -109,6 +109,8 @@ public:
 
 	std::list<InvalidationRecord*> trash;
 
+	static InvalidationRecord* __invalidator (sigc::trackable& trackable, const char*, int);
+
 private:
 	static Glib::Threads::Private<EventLoop> thread_event_loop;
 	std::string _name;
@@ -136,6 +138,6 @@ private:
 
 }
 
-#define MISSING_INVALIDATOR 0 // used to mark places where we fail to provide an invalidator
+#define MISSING_INVALIDATOR nullptr // used to mark places where we fail to provide an invalidator
 
 #endif /* __pbd_event_loop_h__ */