--- a/servo/components/style/rule_tree/mod.rs
+++ b/servo/components/style/rule_tree/mod.rs
@@ -1200,17 +1200,17 @@ impl StrongRuleNode {
         SelfAndAncestors {
             current: Some(self),
         }
     }
 
     /// Returns whether this node has any child, only intended for testing
     /// purposes, and called on a single-threaded fashion only.
     pub unsafe fn has_children_for_testing(&self) -> bool {
-        self.get().children.read().is_empty()
+        !self.get().children.read().is_empty()
     }
 
     unsafe fn pop_from_free_list(&self) -> Option<WeakRuleNode> {
         // NB: This can run from the root node destructor, so we can't use
         // `get()`, since it asserts the refcount is bigger than zero.
         let me = &*self.p.as_ptr();
 
         debug_assert!(me.is_root());