diff --git a/src/core/pbuf.c b/src/core/pbuf.c
index bbc98460..aafa8e41 100644
--- a/src/core/pbuf.c
+++ b/src/core/pbuf.c
@@ -360,12 +360,12 @@ pbuf_header(struct pbuf *p, s16_t header_size_increment)
   if ((header_size_increment == 0) || (p == NULL))
     return 0;
  
-  increment_magnitude = header_size_increment;
-  if (increment_magnitude < 0){
-    increment_magnitude = -increment_magnitude;
+  if (header_size_increment < 0){
+    increment_magnitude = -header_size_increment;
     /* Check that we aren't going to move off the end of the pbuf */
     LWIP_ERROR("increment_magnitude > p->len", (increment_magnitude > p->len), return 1;);
   } else {
+    increment_magnitude = header_size_increment;
 #if 0
     /* Can't assert these as some callers speculatively call
          pbuf_header() to see if it's OK.  Will return 1 below instead. */