gstricklind on "Shortcode content appearing before p tag"

I'm working on a simple column short code plugin. The first paragraph within a short code isn't wrapped in the p tag, but the following paragraph is. I've read this article about problems with wp_autop, and searched google but I can't seem to figure out a solution or why this is happening.

[one_half]paragraph

paragraph[/one_half]
[one_half_last]paragraph

paragraph[/one_half_last]

Without any modification with wp_autop or anything, the html output looks like this:

<div class="one_half">
  paragraph
  <p></p>
  <p>paragraph</p>
</div>
<div class="one_half last">
  paragraph
  <p></p>
  <p>paragraph</p>
</div>

How can I get the first paragraph to be wrapped inside that p tag with proper coding?