diff --git a/libs/ardour/route.cc b/libs/ardour/route.cc
index 573660d038..0614ab7ef5 100644
--- a/libs/ardour/route.cc
+++ b/libs/ardour/route.cc
@@ -2161,10 +2161,14 @@ Route::configure_processors_unlocked (ProcessorStreams* err)
 
 		boost::shared_ptr<PluginInsert> pi;
 		if ((pi = boost::dynamic_pointer_cast<PluginInsert>(*p)) != 0) {
-			/* plugins connected via Split Match may have more channels.
-			 * route/scratch buffers are needed for all of them*/
+			/* plugins connected via Split or Hide Match may have more channels.
+			 * route/scratch buffers are needed for all of them
+			 * The configuration may only be a subset (both input and output)
+			 */
 			processor_max_streams = ChanCount::max(processor_max_streams, pi->input_streams());
+			processor_max_streams = ChanCount::max(processor_max_streams, pi->output_streams());
 			processor_max_streams = ChanCount::max(processor_max_streams, pi->natural_input_streams());
+			processor_max_streams = ChanCount::max(processor_max_streams, pi->natural_output_streams());
 		}
 		out = c->second;