Index: uspace/drv/bus/usb/xhci/transfers.c
===================================================================
--- uspace/drv/bus/usb/xhci/transfers.c	(revision 1af4c002fefa9e2efe75a0780dd018c77b2f7ae9)
+++ uspace/drv/bus/usb/xhci/transfers.c	(revision 4cc0c2e0b29341ba5dfe06af584148b2b1de044b)
@@ -234,4 +234,8 @@
 
 		xhci_trb_ring_t* ring = get_ring(hc, transfer);
+		if (!ring) {
+			return EINVAL;
+		}
+
 		int err = xhci_trb_ring_enqueue(ring, &trb, &transfer->interrupt_trb_phys);