diff --git a/docs/marks/axis.md b/docs/marks/axis.md
index c792615cea..deabd9e519 100644
--- a/docs/marks/axis.md
+++ b/docs/marks/axis.md
@@ -279,7 +279,7 @@ Plot.plot({
   marks: [
     Plot.frame(),
     Plot.dot(penguins, {x: "culmen_length_mm", y: "culmen_depth_mm", fx: "sex", fy: "species"}),
-    Plot.axisX({color: "red", anchor, facetAnchor: facetAnchor === "auto" ? undefined : facetAnchor}),
+    Plot.axisX({color: "red", anchor, facetAnchor: facetAnchor === "auto" ? undefined : facetAnchor === "null" ? null : facetAnchor}),
     Plot.axisFx({color: "blue", anchor: anchor === "top" ? "bottom" : "top"}) // place fx axis opposite x
   ]
 })