Undeclared_variable error thrown in Atom feed

My Atom feed is throwing an odd error that nothing I do (including reinstalling the PHP from scratch) seems to fix. Is something amiss in my database?
Text of error:
<summary type=”text/html”>
<b>Notice</b>: Undefined variable: excerpt in <b>/home/cavlec/public_html/cavlec/wp-includes/template-functions-post.php</b> on line <b>89</b>