diff --git a/kern/kern_xio.c b/kern/kern_xio.c
--- a/kern/kern_xio.c
+++ b/kern/kern_xio.c
@@ -144,6 +144,8 @@
 {
     int i;
 
+    if (npages < 0)
+	    npages = 0;
     KKASSERT(npages <= XIO_INTERNAL_PAGES);
 
     xio->xio_flags = xflags;