diff --git a/demo/cmd.c b/demo/cmd.c
index 2934341..cd574e0 100644
--- a/demo/cmd.c
+++ b/demo/cmd.c
@@ -62,6 +62,7 @@ int main(int argc, char *argv[])
 	struct parsed_options options;
 
 	init_parsed_options(&options);
+	options.subdevice = -1;
 	parse_options(&options, argc, argv);
 
 	/* The following variables used in this demo
@@ -84,6 +85,17 @@ int main(int argc, char *argv[])
 		exit(1);
 	}
 
+	if(options.subdevice < 0) {
+		/* Subdevice not set on command line. */
+		/* Default to the 'read' subdevice (if any). */
+		options.subdevice = comedi_get_read_subdevice(dev);
+		if(options.subdevice < 0) {
+			/* No 'read' subdevice, so default to 0 instead. */
+			options.subdevice = 0;
+		}
+		fprintf(stderr, "defaulted to subdevice %d\n", options.subdevice);
+	}
+
 	// Print numbers for clipped inputs
 	comedi_set_global_oor_behavior(COMEDI_OOR_NUMBER);