diff --git a/ports/unix/netif/tcpdump.c b/ports/unix/netif/tcpdump.c
index d009641..fe108bb 100644
--- a/ports/unix/netif/tcpdump.c
+++ b/ports/unix/netif/tcpdump.c
@@ -124,7 +124,7 @@ tcpdump(struct pbuf *p)
 	    (int)(ntohl(iphdr->dest.addr) >> 8) & 0xff,
 	    (int)(ntohl(iphdr->dest.addr) >> 0) & 0xff,
 	    ntohs(tcphdr->dest));
-    offset = TCPH_OFFSET(tcphdr) >> 4;
+    offset = TCPH_HDRLEN(tcphdr);
 
     len = ntohs(IPH_LEN(iphdr)) - offset * 4 - IP_HLEN;
     if (len != 0 || flags[0] != '.') {