diff --git a/components/script/dom/webidls/Performance.webidl b/components/script/dom/webidls/Performance.webidl
index 0e2f7a960d5b..7a1f9ef1f058 100644
--- a/components/script/dom/webidls/Performance.webidl
+++ b/components/script/dom/webidls/Performance.webidl
@@ -47,7 +47,7 @@ partial interface Performance {
 // https://dvcs.w3.org/hg/webperf/raw-file/tip/specs/NavigationTiming/Overview.html#performance-timing-attribute
 [Exposed=(Window)]
 partial interface Performance {
-  PerformanceNavigationTiming timing();
+  readonly attribute PerformanceNavigationTiming timing;
 };
 // https://w3c.github.io/navigation-timing/#extensions-to-the-performance-interface
 partial interface Performance {