diff --git a/tcpip.cc b/tcpip.cc
index 36befb3ea..c2b6ce707 100644
--- a/tcpip.cc
+++ b/tcpip.cc
@@ -1012,7 +1012,7 @@ unsigned short in_cksum(u16 *ptr,int nbytes) {
    sections 3.2, 11.3, and 17.3. */
 unsigned short magic_tcpudp_cksum(const struct in_addr *src,
 				  const struct in_addr *dst,
-				  u8 proto, u16 len, char *hstart)
+				  u8 proto, u16 len, const void *hstart)
 {
 	struct pseudo {
 		struct in_addr src;
diff --git a/tcpip.h b/tcpip.h
index 1b26f5819..26897f2a5 100644
--- a/tcpip.h
+++ b/tcpip.h
@@ -481,7 +481,7 @@ unsigned short in_cksum(u16 *ptr,int nbytes);
 
 unsigned short magic_tcpudp_cksum(const struct in_addr *src,
 				  const struct in_addr *dst,
-				  u8 proto, u16 len, char *hstart);
+				  u8 proto, u16 len, const void *hstart);
 
 /* Build and send a raw tcp packet.  If TTL is -1, a partially random
    (but likely large enough) one is chosen */