diff --git a/altair/vegalite/v5/api.py b/altair/vegalite/v5/api.py
index be8bf1a89..a5e280874 100644
--- a/altair/vegalite/v5/api.py
+++ b/altair/vegalite/v5/api.py
@@ -1055,7 +1055,7 @@ def to_html(
             requirejs=requirejs,
         )
 
-    def to_url(self, fullscreen: bool = False) -> str:
+    def to_url(self, *, fullscreen: bool = False) -> str:
         """Convert a chart to a URL that opens the chart specification in the Vega chart editor
         The chart specification (including any inline data) is encoded in the URL.