Index: wp-includes/wp-db.php
===================================================================
--- wp-includes/wp-db.php	(revision 11556)
+++ wp-includes/wp-db.php	(working copy)
@@ -396,7 +396,7 @@
 	 */
 	function set_prefix($prefix) {
 
-		if ( preg_match('|[^a-z0-9_]|i', $prefix) )
+		if ( preg_match('|^[^a-z0-9_]*$|i', $prefix) )
 			return new WP_Error('invalid_db_prefix', /*WP_I18N_DB_BAD_PREFIX*/'Invalid database prefix'/*/WP_I18N_DB_BAD_PREFIX*/);
 
 		$old_prefix = $this->prefix;