Index: wp-includes/classes.php
===================================================================
--- wp-includes/classes.php	(revision 3193)
+++ wp-includes/classes.php	(working copy)
@@ -82,7 +82,7 @@
 
 		if ('404' == $qv['error']) {
 			$this->is_404 = true;
-			if ( !empty($query) ) {
+			if ( !empty($query) ) { 
 				do_action('parse_query', array(&$this));
 			}
 			return;
@@ -1612,7 +1612,7 @@
 		// issue a 404 if one was already issued, if the request was a search,
 		// or if the request was a regular query string request rather than a
 		// permalink request.
-		if ( (0 == count($wp_query->posts)) && !is_404() && !is_search() && ( $this->did_permalink || (!empty($_SERVER['QUERY_STRING']) && (false === strpos($_SERVER['REQUEST_URI'], '?'))) ) ) {
+		if ( (0 == count($wp_query->posts)) && !is_404() && !is_category() && !is_search() && ( $this->did_permalink || (!empty($_SERVER['QUERY_STRING']) && (false === strpos($_SERVER['REQUEST_URI'], '?'))) ) ) {
 			$wp_query->set_404();
 			status_header( 404 );
 		}	elseif( is_404() != true ) {