[HTTPS-Everywhere] error messages from HTTPS using proxy

[John]

I am seeing a lot of the following error message in Firefox's error console 
while using the proxy feature of this site 
(https://startpage.com/proxy/uk/help.html), which I expect to use frequently 
from now on:

HTTPS Everywhere: no loadgroup notificationCallbacks for 
https://s7-us2.ixquick-proxy.com/cgi-bin/spg/ccspacer?anticache=457

I'm not really sure what that means, but any ideas what is causing it or how 
to avoid it? Thanks.