diff --git a/components/devtools_traits/lib.rs b/components/devtools_traits/lib.rs
index 4edf19d6f02..b7711821112 100644
--- a/components/devtools_traits/lib.rs
+++ b/components/devtools_traits/lib.rs
@@ -145,7 +145,7 @@ pub struct TimelineMarker {
     pub end_stack: Option<Vec<()>>,
 }
 
-#[derive(PartialEq, Eq, Hash, Clone, Deserialize, Serialize)]
+#[derive(PartialEq, Eq, Hash, Clone, Deserialize, Serialize, HeapSizeOf)]
 pub enum TimelineMarkerType {
     Reflow,
     DOMEvent,