diff --git a/libs/ardour/ardour/session.h b/libs/ardour/ardour/session.h
index 3d45c46ce8..81a5bf02f0 100644
--- a/libs/ardour/ardour/session.h
+++ b/libs/ardour/ardour/session.h
@@ -357,7 +357,8 @@ public:
 	template<class A>            void foreach_route (void (Route::*method)(A), A arg);
 	template<class A1, class A2> void foreach_route (void (Route::*method)(A1, A2), A1 arg1, A2 arg2);
 
-	static char session_name_is_legal (const std::string&);
+	static const std::string session_name_is_legal (const std::string&);
+
 	bool io_name_is_legal (const std::string&) const;
 	boost::shared_ptr<Route> route_by_name (std::string) const;
 	boost::shared_ptr<Route> route_by_id (PBD::ID) const;
diff --git a/libs/ardour/midi_region.cc b/libs/ardour/midi_region.cc
index 5d6c774cc3..fa963fefbd 100644
--- a/libs/ardour/midi_region.cc
+++ b/libs/ardour/midi_region.cc
@@ -828,7 +828,7 @@ MidiRegion::set_name (const std::string& str)
 		return true;
 	}
 
-	if (Session::session_name_is_legal (str) != 0) {
+	if (!Session::session_name_is_legal (str).empty ()) {
 		return false;
 	}
 
diff --git a/libs/ardour/session.cc b/libs/ardour/session.cc
index b0c65463ab..8a3b86ed4f 100644
--- a/libs/ardour/session.cc
+++ b/libs/ardour/session.cc
@@ -6760,18 +6760,23 @@ Session::update_latency_compensation (bool force_whole_graph, bool called_from_b
 	DEBUG_TRACE (DEBUG::LatencyCompensation, "update_latency_compensation: complete\n");
 }
 
-char
+const std::string
 Session::session_name_is_legal (const string& path)
 {
-	char illegal_chars[] = { '/', '\\', ':', ';', '\0' };
+	char illegal_chars[] = { '/', '\\', ':', ';' };
 
 	for (int i = 0; illegal_chars[i]; ++i) {
 		if (path.find (illegal_chars[i]) != string::npos) {
-			return illegal_chars[i];
+			return std::string (1, illegal_chars[i]);
 		}
 	}
 
-	return 0;
+	for (size_t i = 0; i < path.length(); ++i)  {
+		if (iscntrl (path[i])) {
+			return _("Control Char");
+		}
+	}
+	return std::string ();
 }
 
 void
diff --git a/libs/surfaces/osc/osc.cc b/libs/surfaces/osc/osc.cc
index 7998a9d611..c153c50acc 100644
--- a/libs/surfaces/osc/osc.cc
+++ b/libs/surfaces/osc/osc.cc
@@ -2933,9 +2933,9 @@ OSC::name_session (char *n, lo_message msg)
 		return -1;
 	}
 	string new_name = n;
-	char illegal = Session::session_name_is_legal (new_name);
+	std::string const& illegal = Session::session_name_is_legal (new_name);
 
-	if (illegal) {
+	if (!illegal.empty()) {
 		PBD::warning  << (string_compose (_("To ensure compatibility with various systems\n"
 				    "session names may not contain a '%1' character"), illegal)) << endmsg;
 		return -1;