diff --git a/cansend.c b/cansend.c
index b912e80..b8baf58 100644
--- a/cansend.c
+++ b/cansend.c
@@ -75,11 +75,14 @@ int main(int argc, char **argv)
 	required_mtu = parse_canframe(argv[2], &frame);
 	if (!required_mtu){
 		fprintf(stderr, "\nWrong CAN-frame format! Try:\n\n");
-		fprintf(stderr, "    <can_id>#{R|data}          for CAN 2.0 frames\n");
+		fprintf(stderr, "    <can_id>#{data}            for 'classic' CAN 2.0 data frames\n");
+		fprintf(stderr, "    <can_id>#R{len}            for 'classic' CAN 2.0 RTR frames\n");
 		fprintf(stderr, "    <can_id>##<flags>{data}    for CAN FD frames\n\n");
 		fprintf(stderr, "<can_id> can have 3 (SFF) or 8 (EFF) hex chars\n");
 		fprintf(stderr, "{data} has 0..8 (0..64 CAN FD) ASCII hex-values (optionally");
 		fprintf(stderr, " separated by '.')\n");
+		fprintf(stderr, "{len} is an optional 0..8 value as RTR frames can contain a");
+		fprintf(stderr, " valid dlc field\n");
 		fprintf(stderr, "<flags> a single ASCII Hex value (0 .. F) which defines");
 		fprintf(stderr, " canfd_frame.flags\n\n");
 		fprintf(stderr, "e.g. 5A1#11.2233.44556677.88 / 123#DEADBEEF / 5AA# / ");