--- a/wps/wps_registrar.c
+++ b/wps/wps_registrar.c
@@ -1469,7 +1469,7 @@
 
 	wpa_printf(MSG_DEBUG, "WPS: Building Message M4");
 
-	wpa_printf(MSG_DEBUG, "WPS: Dev Password Len: %d", wps->dev_password_len);
+	wpa_printf(MSG_DEBUG, "WPS: Dev Password Len: %zd", wps->dev_password_len);
 	wpa_printf(MSG_DEBUG, "WPS: Dev Password: %s", wps->dev_password);
 
 	wps_derive_psk(wps, wps->dev_password, wps->dev_password_len);