--- a/servo/components/style/stylesheet_set.rs
+++ b/servo/components/style/stylesheet_set.rs
@@ -362,16 +362,19 @@ where
         // Inserting stylesheets somewhere but at the end changes the validity
         // of the cascade data, but not the invalidation data.
         self.set_data_validity_at_least(OriginValidity::CascadeInvalid);
         self.entries.insert(index, StylesheetSetEntry::new(sheet));
     }
 
     fn set_data_validity_at_least(&mut self, validity: OriginValidity) {
         use std::cmp;
+
+        debug_assert_ne!(validity, OriginValidity::Valid);
+
         self.dirty = true;
         self.data_validity = cmp::max(validity, self.data_validity);
     }
 
     fn prepend(&mut self, sheet: S) {
         debug_assert!(!self.contains(&sheet));
         // Inserting stylesheets somewhere but at the end changes the validity
         // of the cascade data, but not the invalidation data.