diff --git a/src/tun.c b/src/tun.c
index 65ea125..8130b5c 100644
--- a/src/tun.c
+++ b/src/tun.c
@@ -224,7 +224,7 @@ tun_write(int fd, const void *data, size_t size)
 #ifdef GT_BSD_TUN
     uint32_t family;
 
-    switch (ip_get_version(data)) {
+    switch (ip_get_version(data, (int)size)) {
     case 4:
         family = htonl(AF_INET);
         break;