diff --git a/man2/add_key.2 b/man2/add_key.2
index 2214bd91a..64e5e56bf 100644
--- a/man2/add_key.2
+++ b/man2/add_key.2
@@ -13,6 +13,7 @@
 add_key \- add a key to the kernel's key management facility
 .SH SYNOPSIS
 .nf
+.B #include <sys/types.h>
 .B #include <keyutils.h>
 .sp
 .BI "key_serial_t add_key(const char *" type ", const char *" description ,