diff --git a/web-animations-1/Overview.bs b/web-animations-1/Overview.bs
index a9d827441d0..806af1a22ee 100644
--- a/web-animations-1/Overview.bs
+++ b/web-animations-1/Overview.bs
@@ -6100,7 +6100,7 @@ The {{KeyframeEffect}} interface {#the-keyframeeffect-interface}
       (CompositeOperationOrAuto or sequence<CompositeOperationOrAuto>) composite = [];
     };
 
-    typedef (sequence<Keyframe> or PropertyIndexedKeyframes) KeyframeArgument;
+    typedef (sequence<Keyframe?> or PropertyIndexedKeyframes) KeyframeArgument;
   </xmp>
 
   The meaning and allowed values of each argument is as follows: