How about setting it to this:
<a href="[[~[[++site_start]]]]">
(assuming that it's Revolution).
For Evolution, I think it would be this, but I can't remember for sure:
My guess on the snippet is that the part that returns the URL is inside an if statement that's not always true. When it fails, the snippet falls through to the end where there's no return statement and it returns the default value for a successfully completed script, which is 1.