diff --git a/net/decnet/dn_dev.c b/net/decnet/dn_dev.c
index bfd43e8f2c06e8c1c8e69d8ce95a349734166584..3235540f6adff0a924a06c36c7b219e76e7167cb 100644
--- a/net/decnet/dn_dev.c
+++ b/net/decnet/dn_dev.c
@@ -56,7 +56,7 @@
 #include <net/dn_neigh.h>
 #include <net/dn_fib.h>
 
-#define DN_IFREQ_SIZE (sizeof(struct ifreq) - sizeof(struct sockaddr) + sizeof(struct sockaddr_dn))
+#define DN_IFREQ_SIZE (offsetof(struct ifreq, ifr_ifru) + sizeof(struct sockaddr_dn))
 
 static char dn_rt_all_end_mcast[ETH_ALEN] = {0xAB,0x00,0x00,0x04,0x00,0x00};
 static char dn_rt_all_rt_mcast[ETH_ALEN]  = {0xAB,0x00,0x00,0x03,0x00,0x00};