Dummynet + Iperf client conflict

Has anyone tried to run Dummynet impairment at the same time as an Iperf client?
They seem to conflict with Iperf giving error “write failed: Interrupted function call”.

I have a Iperf server running on a separate PC. Then on the Dummynet PC I enter the following:
ipfw add prob 0.1 deny proto udp out
iperf -c -u -b1M

Iperf then gives the “write failed: Interrupted function call” error.
If I omit the ipfw command the iperf transfer works.
Using Dummynet on the same PC as an Iperf server instead of Iperf client works but that’s not what I want.