diff --git a/lib/prefix.c b/lib/prefix.c
index 1a4a914e05..053e4533e7 100644
--- a/lib/prefix.c
+++ b/lib/prefix.c
@@ -441,7 +441,7 @@ void prefix_hexdump(const struct prefix *p)
 	zlog_hexdump(p, sizeof(struct prefix));
 }
 
-int is_zero_mac(struct ethaddr *mac)
+int is_zero_mac(const struct ethaddr *mac)
 {
 	int i = 0;
 
diff --git a/lib/prefix.h b/lib/prefix.h
index e338140f1a..24c146e022 100644
--- a/lib/prefix.h
+++ b/lib/prefix.h
@@ -470,7 +470,7 @@ extern void masklen2ip6(const int, struct in6_addr *);
 
 extern const char *inet6_ntoa(struct in6_addr);
 
-extern int is_zero_mac(struct ethaddr *mac);
+extern int is_zero_mac(const struct ethaddr *mac);
 extern int prefix_str2mac(const char *str, struct ethaddr *mac);
 extern char *prefix_mac2str(const struct ethaddr *mac, char *buf, int size);