Filters sometimes 'blow up'

Sometimes, when working with filters I've gotten a full volume 'screaming' sound over the speakers or headphones. The volume settings of my laptop don't affect the level of the screaming, it's always at full volume.

Here's code that sends to a blackhole that demonstrates the runaway filter problem:

PulseOsc osc => BPF filter => blackhole;
20000 => filter.freq;
0.5 => filter.Q;
for(int i; i < 100; i++)
{
   <<< "osc: ", osc.last(), " filter: ", filter.last() >>>;
   samp => now;
}

The output of this code keeps rising.

(using a macbook, internal soundcard and dracula version of ChucK)