diff --git a/lib/sockunion.h b/lib/sockunion.h
index 99bdf6a..92e536c 100644
--- a/lib/sockunion.h
+++ b/lib/sockunion.h
@@ -87,7 +87,7 @@
 #define sockunion_family(X)  (X)->sa.sa_family
 
 /* Prototypes. */
-int str2sockunion (char *, union sockunion *);
+int str2sockunion (const char *, union sockunion *);
 const char *sockunion2str (union sockunion *, char *, size_t);
 int sockunion_cmp (union sockunion *, union sockunion *);
 int sockunion_same (union sockunion *, union sockunion *);