--- a/sys/bus/u4b/net/usb_ethernet.h
+++ b/sys/bus/u4b/net/usb_ethernet.h
@@ -102,6 +102,7 @@
 	struct usb_ether_cfg_task	ue_tick_task[2];
 
 	int			ue_unit;
+	int			ue_if_attached;	/* ether_ifattach() ran */
 
 	/* ethernet address from eeprom */
 	uint8_t			ue_eaddr[ETHER_ADDR_LEN];
--- a/sys/bus/u4b/net/usb_ethernet.c
+++ b/sys/bus/u4b/net/usb_ethernet.c
@@ -241,6 +241,7 @@
 
 	if_printf(ifp, "<USB Ethernet> on %s\n", device_get_nameunit(ue->ue_dev));
 	ether_ifattach(ifp, ue->ue_eaddr, NULL);
+	ue->ue_if_attached = 1;
 	/* Tell upper layer we support VLAN oversized frames. */
 	if (ifp->if_capabilities & IFCAP_VLAN_MTU)
 		ifp->if_hdrlen = sizeof(struct ether_vlan_header);
@@ -275,7 +276,9 @@
 	/* read "ifnet" pointer after taskqueue drain */
 	ifp = uether_getifp(ue);
 
-	if (ifp != NULL) {
+	/* the async attach task may have failed before ether_ifattach();
+	 * detaching a never-attached ifnet panics in if_detach() */
+	if (ifp != NULL && ue->ue_if_attached) {
 
 		/* we are not running any more */
 		UE_LOCK(ue);