diff --git a/servers/slapd/back-perl/modify.c b/servers/slapd/back-perl/modify.c
index 237a782de2..e8335cd01e 100644
--- a/servers/slapd/back-perl/modify.c
+++ b/servers/slapd/back-perl/modify.c
@@ -16,6 +16,7 @@
  */
 
 #include "perl_back.h"
+#include <ac/string.h>
 
 int
 perl_back_modify(