diff --git a/lib/prefix.h b/lib/prefix.h
index a1a0679..404a63a 100644
--- a/lib/prefix.h
+++ b/lib/prefix.h
@@ -189,7 +189,7 @@
 
 extern struct prefix *sockunion2prefix (const union sockunion *dest,
                                         const union sockunion *mask);
-extern struct prefix *sockunion2hostprefix (const union sockunion *);
+extern struct prefix *sockunion2hostprefix (const union sockunion *, struct prefix *p);
 extern void prefix2sockunion (const struct prefix *, union sockunion *);
 
 extern struct prefix_ipv4 *prefix_ipv4_new (void);