diff --git a/sys/net/dummynet/ip_dummynet.c b/sys/net/dummynet/ip_dummynet.c
--- a/sys/net/dummynet/ip_dummynet.c
+++ b/sys/net/dummynet/ip_dummynet.c
@@ -1434,6 +1434,8 @@
     x->qsize = ioc_fs->qsize;
     x->plr = ioc_fs->plr;
     set_flowid_parms(&x->flow_mask, &ioc_fs->flow_mask);
+    if (x->qsize < 0)
+	x->qsize = 0;
     if (x->flags_fs & DN_QSIZE_IS_BYTES) {
 	if (x->qsize > 1024 * 1024)
 	    x->qsize = 1024 * 1024;