diff --git a/libs/ardour/ardour/route.h b/libs/ardour/ardour/route.h
index 829bf13298..864ae911e5 100644
--- a/libs/ardour/ardour/route.h
+++ b/libs/ardour/ardour/route.h
@@ -224,8 +224,8 @@ class LIBARDOUR_API Route : public SessionObject, public Automatable, public Rou
 
 	boost::shared_ptr<Processor> processor_by_id (PBD::ID) const;
 
-	boost::shared_ptr<Processor> nth_plugin (uint32_t n);
-	boost::shared_ptr<Processor> nth_send (uint32_t n);
+	boost::shared_ptr<Processor> nth_plugin (uint32_t n) const;
+	boost::shared_ptr<Processor> nth_send (uint32_t n) const;
 
 	bool has_io_processor_named (const std::string&);
 	ChanCount max_processor_streams () const { return processor_max_streams; }
diff --git a/libs/ardour/route.cc b/libs/ardour/route.cc
index 8549bb34a6..ff765ad89d 100644
--- a/libs/ardour/route.cc
+++ b/libs/ardour/route.cc
@@ -4491,10 +4491,10 @@ Route::get_control (const Evoral::Parameter& param)
 }
 
 boost::shared_ptr<Processor>
-Route::nth_plugin (uint32_t n)
+Route::nth_plugin (uint32_t n) const
 {
 	Glib::Threads::RWLock::ReaderLock lm (_processor_lock);
-	ProcessorList::iterator i;
+	ProcessorList::const_iterator i;
 
 	for (i = _processors.begin(); i != _processors.end(); ++i) {
 		if (boost::dynamic_pointer_cast<PluginInsert> (*i)) {
@@ -4508,10 +4508,10 @@ Route::nth_plugin (uint32_t n)
 }
 
 boost::shared_ptr<Processor>
-Route::nth_send (uint32_t n)
+Route::nth_send (uint32_t n) const
 {
 	Glib::Threads::RWLock::ReaderLock lm (_processor_lock);
-	ProcessorList::iterator i;
+	ProcessorList::const_iterator i;
 
 	for (i = _processors.begin(); i != _processors.end(); ++i) {
 		if (boost::dynamic_pointer_cast<Send> (*i)) {