diff --git a/gtp/gtp.h b/gtp/gtp.h
index 3ce89ed..539e255 100644
--- a/gtp/gtp.h
+++ b/gtp/gtp.h
@@ -133,6 +133,7 @@
 
 struct ul66_t;
 struct ul16_t;
+struct pdp_t;
 
 /* GTP 0 header. 
  * Explanation to some of the fields: