Index: sys/dev/xen/grant_table/grant_table.c
===================================================================
--- sys/dev/xen/grant_table/grant_table.c
+++ sys/dev/xen/grant_table/grant_table.c
@@ -184,6 +184,10 @@
 {
 	uint16_t flags, nflags;
 
+	if (ref == GRANT_REF_INVALID) {
+		return (0);
+	}
+
 	nflags = shared[ref].flags;
 	do {
 		if ( (flags = nflags) & (GTF_reading|GTF_writing) ) {
@@ -349,8 +353,10 @@
 	int ref, error;
 
 	error = get_free_entries(count, &ref);
-	if (__predict_false(error))
+	if (__predict_false(error)) {
+		*head = GNTTAB_LIST_END;
 		return (error);
+	}
 
 	*head = ref;
 	return (0);