diff --git a/judge/runguard.cc b/judge/runguard.cc
index 62b54f222f..4b544d8da5 100644
--- a/judge/runguard.cc
+++ b/judge/runguard.cc
@@ -812,7 +812,7 @@ void setrestrictions()
 	/* Set group-id (must be root for this, so before setting user). */
 	if ( use_group ) {
 		if ( setgid(rungid) ) error(errno,"cannot set group ID to `%d'",rungid);
-		gid_t aux_groups[10];
+		gid_t aux_groups[1];
 		aux_groups[0] = rungid;
 		if ( setgroups(1, aux_groups) ) error(errno,"cannot clear auxiliary groups");