diff --git a/sys/dev/usb/usb_pf.c b/sys/dev/usb/usb_pf.c
index 3e6b7d157c9..e4ee6254b7b 100644
--- a/sys/dev/usb/usb_pf.c
+++ b/sys/dev/usb/usb_pf.c
@@ -67,7 +67,7 @@ usbpf_attach(struct usb_bus *ubus)
 	if_attach(ifp);
 
 	KASSERT(sizeof(struct usbpf_pkthdr) == USBPF_HDR_LEN,
-	    ("wrong USB pf header length (%d)", sizeof(struct usbpf_pkthdr)));
+	    ("wrong USB pf header length (%zd)", sizeof(struct usbpf_pkthdr)));
 
 	/*
 	 * XXX According to the specification of DLT_USB, it indicates packets