diff --git a/libs/surfaces/websockets/mixer.cc b/libs/surfaces/websockets/mixer.cc
index 13cc3a32f4..2d794fdb7e 100644
--- a/libs/surfaces/websockets/mixer.cc
+++ b/libs/surfaces/websockets/mixer.cc
@@ -16,6 +16,8 @@
  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
+#include <boost/lexical_cast.hpp>
+
 #include "ardour/dB.h"
 #include "ardour/meter.h"
 #include "ardour/plugin_insert.h"
@@ -99,7 +101,8 @@ ArdourMixerPlugin::param_control (uint32_t param_n) const
 	uint32_t                  control_id = plugin->nth_parameter (param_n, ok);
 
 	if (!ok || !plugin->parameter_is_input (control_id)) {
-		throw ArdourMixerNotFoundException("invalid automation control");
+		throw ArdourMixerNotFoundException("invalid automation control for param id = "
+			+ boost::lexical_cast<std::string>(param_n));
 	}
 
 	return _insert->automation_control (Evoral::Parameter (PluginAutomation, 0, control_id));
@@ -158,7 +161,7 @@ ArdourMixerStrip::nth_plugin (uint32_t plugin_n)
 		return _plugins[plugin_n];
 	}
 
-	throw ArdourMixerNotFoundException (""/*"Plugin with ID " + plugin_n + " not found"*/);
+	throw ArdourMixerNotFoundException ("plugin id = " + boost::lexical_cast<std::string>(plugin_n) + " not found");
 }
 
 double
@@ -178,8 +181,7 @@ ArdourMixerStrip::pan () const
 {
 	boost::shared_ptr<AutomationControl> ac = _stripable->pan_azimuth_control ();
 	if (!ac) {
-		/* TODO: inform GUI that strip has no panner */
-		return 0;
+		throw ArdourMixerNotFoundException ("strip has no panner");
 	}
 	return ac->internal_to_interface (ac->get_value ());
 }
@@ -290,7 +292,7 @@ ArdourMixer::nth_strip (uint32_t strip_n)
 		return _strips[strip_n];
 	}
 
-	throw ArdourMixerNotFoundException (""/*"Strip with ID " + strip_n + " not found"*/);
+	throw ArdourMixerNotFoundException ("strip id = " + boost::lexical_cast<std::string>(strip_id) + " not found");
 }
 
 void