diff --git a/C/unify.c b/C/unify.c
index e0bb0e85e..1e7f9e8d9 100644
--- a/C/unify.c
+++ b/C/unify.c
@@ -26,7 +26,9 @@ STATIC_PROTO(int    rtable_hash_op, (OPCODE));
 STATIC_PROTO(void   InitReverseLookupOpcode, (void));
 #endif
 
-static int
+int STD_PROTO(rational_tree_loop,(CELL *, CELL *, CELL **));
+
+int
 rational_tree_loop(CELL *pt0, CELL *pt0_end, CELL **to_visit0)
 {
   CELL **to_visit = to_visit0;