diff --git a/src/core/ipv4/inet_chksum.c b/src/core/ipv4/inet_chksum.c
index a30a1834..dea8d5f2 100644
--- a/src/core/ipv4/inet_chksum.c
+++ b/src/core/ipv4/inet_chksum.c
@@ -66,7 +66,7 @@
 /** Like the name says... */
 #if LWIP_PLATFORM_BYTESWAP && (BYTE_ORDER == LITTLE_ENDIAN)
 /* little endian and PLATFORM_BYTESWAP defined */
-#define SWAP_BYTES_IN_WORD(w) LWIP_PLATFORM_HTONS(x)
+#define SWAP_BYTES_IN_WORD(w) LWIP_PLATFORM_HTONS(w)
 #else
 /* can't use htons on big endian (or PLATFORM_BYTESWAP not defined)... */
 #define SWAP_BYTES_IN_WORD(w) ((w & 0xff) << 8) | ((w & 0xff00) >> 8)