Index: uspace/lib/usbhost/src/bus.c
===================================================================
--- uspace/lib/usbhost/src/bus.c	(revision d42ba372b0d79f8dd020b8f145190f30d31e8d2f)
+++ uspace/lib/usbhost/src/bus.c	(revision 7d957b20978eb7a4e4741ee6fbda6c0e33c08ddf)
@@ -131,10 +131,10 @@
 	assert(device);
 
-	fibril_mutex_lock(&bus->guard);
-
 	if (desc->max_packet_size == 0 || desc->packets == 0) {
 		usb_log_warning("Invalid endpoint description (mps %zu, %u packets)", desc->max_packet_size, desc->packets);
 		return EINVAL;
 	}
+
+	fibril_mutex_lock(&bus->guard);
 
 	int err = ENOMEM;