diff --git a/bin/rndc/rndc.c b/bin/rndc/rndc.c
index 088cfd1a17..346c06e4b6 100644
--- a/bin/rndc/rndc.c
+++ b/bin/rndc/rndc.c
@@ -121,7 +121,7 @@ command is one of the following:\n\
 		Requires the zone to have a dnssec-policy.\n\
   dnstap -reopen\n\
 		Close, truncate and re-open the DNSTAP output file.\n\
-  dnstap -roll count\n\
+  dnstap -roll [count]\n\
 		Close, rename and re-open the DNSTAP output file(s).\n\
   dumpdb [-all|-cache|-zones|-adb|-bad|-expired|-fail] [view ...]\n\
 		Dump cache(s) to the dump file (named_dump.db).\n\