asterisk users - Dec 2009 - MixMonitor stops audio in SIP to SIP call

Has anyone experienced this problem before?

I am running Asterisk 1.4.21.2
If I run:

MixMonitor(..)
Dial(SIP/...)

Both parties cannot hear each other.
As soon as I comment out MixMonitor, the audio can be heard.

I saw this issue on https://issues.asterisk.org/view.php?id=16256
It seems to match what I encountered.

Any ideas?