Frequently repeated [Warning] Weighted bandwidth is 0.000000 ... when using bridges

When using bridges, the following log message repeats every few seconds:

[Warning] Weighted bandwidth is 0.000000 in node selection for rule weight as guard

I have two bridges configured, both are online and available. Other than the bridges, this is a default install of the vidalia bundle for ubuntu 10.04 running tor 0.2.2.14-alpha.