diff --git a/index.bs b/index.bs
index d93e211..a3e53a0 100644
--- a/index.bs
+++ b/index.bs
@@ -5937,7 +5937,7 @@ interface VTTRegion {
   constructor();
   attribute DOMString id;
   attribute double width;
-  attribute unsigned long lines;
+  attribute long lines;
   attribute double regionAnchorX;
   attribute double regionAnchorY;
   attribute double viewportAnchorX;