diff --git a/src/core/pbuf.c b/src/core/pbuf.c
index 4968404d..40cfca38 100644
--- a/src/core/pbuf.c
+++ b/src/core/pbuf.c
@@ -212,21 +212,21 @@ pbuf_alloc(pbuf_layer layer, u16_t length, pbuf_type type)
   LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE, ("pbuf_alloc(length=%"U16_F")\n", length));
 
   /* determine header offset */
-  offset = 0;
   switch (layer) {
   case PBUF_TRANSPORT:
     /* add room for transport (often TCP) layer header */
-    offset += PBUF_TRANSPORT_HLEN;
-    /* FALLTHROUGH */
+    offset = PBUF_LINK_HLEN + PBUF_IP_HLEN + PBUF_TRANSPORT_HLEN;
+    break;
   case PBUF_IP:
     /* add room for IP layer header */
-    offset += PBUF_IP_HLEN;
-    /* FALLTHROUGH */
+    offset = PBUF_LINK_HLEN + PBUF_IP_HLEN;
+    break;
   case PBUF_LINK:
     /* add room for link layer header */
-    offset += PBUF_LINK_HLEN;
+    offset = PBUF_LINK_HLEN;
     break;
   case PBUF_RAW:
+    offset = 0;
     break;
   default:
     LWIP_ASSERT("pbuf_alloc: bad pbuf layer", 0);
@@ -371,21 +371,21 @@ pbuf_alloced_custom(pbuf_layer l, u16_t length, pbuf_type type, struct pbuf_cust
   LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE, ("pbuf_alloced_custom(length=%"U16_F")\n", length));
 
   /* determine header offset */
-  offset = 0;
   switch (l) {
   case PBUF_TRANSPORT:
     /* add room for transport (often TCP) layer header */
-    offset += PBUF_TRANSPORT_HLEN;
-    /* FALLTHROUGH */
+    offset = PBUF_LINK_HLEN + PBUF_IP_HLEN + PBUF_TRANSPORT_HLEN;
+    break;
   case PBUF_IP:
     /* add room for IP layer header */
-    offset += PBUF_IP_HLEN;
-    /* FALLTHROUGH */
+    offset = PBUF_LINK_HLEN + PBUF_IP_HLEN;
+    break;
   case PBUF_LINK:
     /* add room for link layer header */
-    offset += PBUF_LINK_HLEN;
+    offset = PBUF_LINK_HLEN;
     break;
   case PBUF_RAW:
+    offset = 0;
     break;
   default:
     LWIP_ASSERT("pbuf_alloced_custom: bad pbuf layer", 0);