diff --git a/lib/prefix.c b/lib/prefix.c
index c85e659..7dc866d 100644
--- a/lib/prefix.c
+++ b/lib/prefix.c
@@ -41,7 +41,7 @@
 
 /* Address Famiy Identifier to Address Family converter. */
 int
-afi2family (int afi)
+afi2family (afi_t afi)
 {
   if (afi == AFI_IP)
     return AF_INET;
@@ -52,7 +52,7 @@
   return 0;
 }
 
-int
+afi_t
 family2afi (int family)
 {
   if (family == AF_INET)
diff --git a/lib/prefix.h b/lib/prefix.h
index a7598b7..5f1ff05 100644
--- a/lib/prefix.h
+++ b/lib/prefix.h
@@ -144,8 +144,8 @@
 }
 
 /* Prototypes. */
-extern int afi2family (int);
-extern int family2afi (int);
+extern int afi2family (afi_t);
+extern afi_t family2afi (int);
 
 extern struct prefix *prefix_new (void);
 extern void prefix_free (struct prefix *);