Index: wp-includes/query.php
===================================================================
--- wp-includes/query.php	(revision 17511)
+++ wp-includes/query.php	(working copy)
@@ -2190,7 +2190,7 @@
 				if ( !empty( $cat_query ) ) {
 					$cat_query = reset( $cat_query );
 					$the_cat = get_term_by( $cat_query['field'], $cat_query['terms'][0], 'category' );
-					if ( $the_cat && ! is_wp_error( $the_cat ) ) {
+					if ( $the_cat ) {
 						$this->set( 'cat', $the_cat->term_id );
 						$this->set( 'category_name', $the_cat->slug );
 					}
@@ -2202,7 +2202,7 @@
 				if ( !empty( $tag_query ) ) {
 					$tag_query = reset( $tag_query );
 					$the_tag = get_term_by( $tag_query['field'], $tag_query['terms'][0], 'post_tag' );
-					if ( $the_tag && ! is_wp_error( $the_tag ) ) {
+					if ( $the_tag ) {
 						$this->set( 'tag_id', $the_tag->term_id );
 					}
 					unset( $the_tag );
Index: wp-includes/taxonomy.php
===================================================================
--- wp-includes/taxonomy.php	(revision 17511)
+++ wp-includes/taxonomy.php	(working copy)
@@ -887,7 +887,10 @@
 		$value = stripslashes($value);
 		$field = 't.name';
 	} else {
-		return get_term( (int) $value, $taxonomy, $output, $filter);
+		$term = get_term( (int) $value, $taxonomy, $output, $filter);
+		if ( is_wp_error( $term ) )
+			$term = false;
+		return $term;
 	}
 
 	$term = $wpdb->get_row( $wpdb->prepare( "SELECT t.*, tt.* FROM $wpdb->terms AS t INNER JOIN $wpdb->term_taxonomy AS tt ON t.term_id = tt.term_id WHERE tt.taxonomy = %s AND $field = %s LIMIT 1", $taxonomy, $value) );