diff --git a/etc/layout_viewer/README b/etc/layout_viewer/README
index ee0b065a87b..11c765e0a74 100644
--- a/etc/layout_viewer/README
+++ b/etc/layout_viewer/README
@@ -1,6 +1,6 @@
 To generate a layout trace:
 
-./servo --trace-layout url
+./mach run -Z trace-layout <url>
 
 A layout_trace.json file will be writtent to the current directory that can be loaded by the viewer.