[Observium] Cisco ASA Remote Access Sessions getting Error Drawing Graph

Using Observium to monitor several pairs of Cisco ASA 5520 firewalls. On the graphs/firewall page

/observium/device/device=27/tab=graphs/group=firewall/

the very last graph is labeled Remote Access Sessions. I'm getting the ubiquitous "Error Drawing Graph" on that row of graphs.

whats my next debugging step? The apache logs don't show any errors, so I'm guessing its somewhere in the Observium or RRDtool layers.

Observium 0.11.11.2726 Apache 2.2.14 (Ubuntu) PHP 5.3.2-1ubuntu4.10 MySQL 5.1.41-3ubuntu12.10

Thanks

---- Jeff