diff --git a/sys-utils/dmesg.c b/sys-utils/dmesg.c
index e6819308c..88713d55d 100644
--- a/sys-utils/dmesg.c
+++ b/sys-utils/dmesg.c
@@ -1364,6 +1364,9 @@ int main(int argc, char *argv[])
 		case DMESG_TIMEFTM_TIME:
 			ctl.time_fmt = DMESG_TIMEFTM_TIME_DELTA;
 			break;
+		case DMESG_TIMEFTM_ISO8601:
+			warnx(_("--show-delta is ignored when used together with iso8601 time format"));
+			break;
 		default:
 			ctl.time_fmt = DMESG_TIMEFTM_DELTA;
 		}