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
@@ -1343,6 +1343,8 @@
     x->max_th = SCALE(ioc_fs->max_th);
     x->max_p = ioc_fs->max_p;
 
+    if (ioc_fs->max_th <= ioc_fs->min_th)
+	return EINVAL;
     x->c_1 = ioc_fs->max_p / (ioc_fs->max_th - ioc_fs->min_th);
     x->c_2 = SCALE_MUL(x->c_1, SCALE(ioc_fs->min_th));
     if (x->flags_fs & DN_IS_GENTLE_RED) {