--- a/servo/src/servo/dom/rcu.rs
+++ b/servo/src/servo/dom/rcu.rs
@@ -51,17 +51,17 @@ impl reader_methods<T:send,A> for handle
         self.rd_aux().is_not_null()
     }
 
     fn set_aux(p: @A) unsafe {
         (**self).rd_aux = ptr::addr_of(*p);
     }
 
     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())
     }
 }
 
 impl private_methods<T:send,A> for scope<T,A> {
     fn clone(v: *T) -> *T unsafe {
         let n: *mut T =
             unsafe::reinterpret_cast(
@@ -235,9 +235,9 @@ mod test {
             }
             s.reader_joined();
         }
 
         assert henrietta.rd(read_characteristic) == iter1 * iter2;
         assert ferdinand.rd(read_characteristic) == iter1 * iter2;
     }
 
-}
\ No newline at end of file
+}