[Plugin: WordPress SEO by Yoast] excerpt not auto-generated

WordPress version: 3.4
WordPress SEO version: 1.2.3
Using Atahualpa 3.7.7.

I did this:
I used %%excerpt%% in a pages Meta description template.

I expected the plugin to do this:
I expected it to generate an excerpt from the text on my page and use that in the meta description.

Instead it did this:
%%excerpt%% was not replaced with an auto-generated excerpt but with spaces.

Is this because of a lack of understanding from my part, or is this a bug?

I noticed a similar thread by kanuxdesigner from 1 week ago, but no replies there. Hope this one does…

https://www.ads-software.com/extend/plugins/wordpress-seo/