Google Plus One script has quit working, now returns red flag

Question

Here's the Google Plus One script I've been using:

<g:plusone a href="<?php echo urlencode(get_permalink($post->ID)); ?>"></g:plusone>

Worked fine until it didn't. Now it's returning a red Plus One button with a bunch of suggestions to correct the error, none of which apply.

Tried out the Plus One plugin, which works on single posts and pages, but screws up on custom post type pages. I'd rather not use a plugin.

Does anyone have a Plus One script that will work consistently with WordPress?

Thanks.