diff --git a/src/core/init.c b/src/core/init.c
index 22d5616c..dbd630dd 100644
--- a/src/core/init.c
+++ b/src/core/init.c
@@ -201,7 +201,7 @@
   #error "LWIP_ETHERNET needs to be turned on for LWIP_ARP or PPPOE_SUPPORT"
 #endif
 #if (LWIP_IGMP || LWIP_IPV6) && !defined(LWIP_RAND)
-  #error "When using IGMP or IPv6, LWIP_RAND() needs to be defined to a random-function returning an u32_t random value"
+  #error "When using IGMP or IPv6, LWIP_RAND() needs to be defined to a random-function returning an u32_t random value (in arch/cc.h)"
 #endif
 #if LWIP_TCPIP_CORE_LOCKING_INPUT && !LWIP_TCPIP_CORE_LOCKING
   #error "When using LWIP_TCPIP_CORE_LOCKING_INPUT, LWIP_TCPIP_CORE_LOCKING must be enabled, too"