diff --git a/src/core/ipv4/igmp.c b/src/core/ipv4/igmp.c
index 22a1e161..bd52744f 100644
--- a/src/core/ipv4/igmp.c
+++ b/src/core/ipv4/igmp.c
@@ -699,8 +699,10 @@ igmp_start_timer(struct igmp_group *group, u8_t max_time)
   if (max_time == 0) {
     max_time = 1;
   }
+#ifdef LWIP_RAND
   /* ensure the random value is > 0 */
   group->timer = (LWIP_RAND() % (max_time - 1)) + 1;
+#endif /* LWIP_RAND */
 }
 
 /**
diff --git a/src/core/ipv6/mld6.c b/src/core/ipv6/mld6.c
index 9db25bab..1cb2dd9c 100644
--- a/src/core/ipv6/mld6.c
+++ b/src/core/ipv6/mld6.c
@@ -493,8 +493,10 @@ mld6_delayed_report(struct mld_group *group, u16_t maxresp)
     maxresp = 1;
   }
 
-  /* Randomize maxresp. */
+#ifdef LWIP_RAND
+  /* Randomize maxresp. (if LWIP_RAND is supported) */
   maxresp = (LWIP_RAND() % (maxresp - 1)) + 1;
+#endif /* LWIP_RAND */
 
   /* Apply timer value if no report has been scheduled already. */
   if ((group->group_state == MLD6_GROUP_IDLE_MEMBER) ||