diff --git a/components/selectors/gecko_like_types.rs b/components/selectors/gecko_like_types.rs
index 9ffffaf2aed..ac003151df4 100644
--- a/components/selectors/gecko_like_types.rs
+++ b/components/selectors/gecko_like_types.rs
@@ -17,6 +17,7 @@ pub enum PseudoClass {
 pub enum PseudoElement {
     A,
     B,
+    Tree(Box<[String]>),
 }
 
 #[derive(Eq, PartialEq, Clone, Debug, Default)]
diff --git a/components/selectors/size_of_tests.rs b/components/selectors/size_of_tests.rs
index 66ce80b8d64..46b966ec5f3 100644
--- a/components/selectors/size_of_tests.rs
+++ b/components/selectors/size_of_tests.rs
@@ -12,7 +12,7 @@ use std::fmt;
 use visitor::SelectorVisitor;
 
 size_of_test!(size_of_selector, Selector<Impl>, 8);
-size_of_test!(size_of_pseudo_element, gecko_like_types::PseudoElement, 1);
+size_of_test!(size_of_pseudo_element, gecko_like_types::PseudoElement, 24);
 
 size_of_test!(size_of_component, Component<Impl>, 32);
 size_of_test!(size_of_pseudo_class, PseudoClass, 24);