diff --git a/src/servo/dom/rcu.rs b/src/servo/dom/rcu.rs
index f54fe0c5a6e..df78116506b 100644
--- a/src/servo/dom/rcu.rs
+++ b/src/servo/dom/rcu.rs
@@ -56,7 +56,7 @@ impl reader_methods<T:send,A> for handle<T,A> {
     }
 
     fn aux<U>(f: fn(A) -> U) -> U unsafe {
-        // warning: do not use if has_aux() is false!
+        assert self.has_aux();
         f(*self.rd_aux())
     }
 }
@@ -240,4 +240,4 @@ mod test {
         assert ferdinand.rd(read_characteristic) == iter1 * iter2;
     }
 
-}
\ No newline at end of file
+}