diff --git a/css-typed-om/Overview.bs b/css-typed-om/Overview.bs
index e3649532..2de05c0c 100644
--- a/css-typed-om/Overview.bs
+++ b/css-typed-om/Overview.bs
@@ -2743,7 +2743,9 @@ objects, the {{CSSURLImageValue/url}} attribute contains the URL that references
     when called,
     perform the following steps:
 
-    1. Return a new {{CSSURLImageValue}}
+    1. If the |url| passed into the constructor is not a [[url#valid-url-string]],
+        throw a {{TypeError}} and exit this algorithm.
+    2. Else, return a new {{CSSURLImageValue}}
         with its {{CSSURLImageValue/url}} internal slot
         set to |url|.
 </div>