--- wp-includes/wp-db.php	(revision 26860)
+++ wp-includes/wp-db.php	(working copy)
@@ -171,6 +171,15 @@
 	var $prefix = '';
 
 	/**
+	 * WordPress base table prefix
+	 *
+	 * @since 3.0
+	 * @access public
+	 * @var int
+	 */
+	 var $base_prefix;
+
+	/**
 	 * Whether the database queries are ready to start executing.
 	 *
 	 * @since 2.3.2
@@ -664,7 +673,7 @@
 
 		$old_prefix = is_multisite() ? '' : $prefix;
 
-		if ( isset( $this->base_prefix ) )
+		if ( !empty( $this->base_prefix ) )
 			$old_prefix = $this->base_prefix;
 
 		$this->base_prefix = $prefix;