diff --git a/core/compiler/units/units.factor b/core/compiler/units/units.factor
index 7c4116c9f4..ea39e82239 100644
--- a/core/compiler/units/units.factor
+++ b/core/compiler/units/units.factor
@@ -2,8 +2,8 @@
 ! See http://factorcode.org/license.txt for BSD license.
 USING: accessors arrays assocs classes classes.private
 classes.tuple classes.tuple.private continuations definitions
-generic init kernel kernel.private math namespaces sequences
-sets source-files.errors vocabs words ;
+generic hash-sets init kernel kernel.private math namespaces
+sequences sets source-files.errors vocabs words ;
 FROM: namespaces => set ;
 FROM: sets => members ;
 IN: compiler.units
@@ -168,7 +168,7 @@ M: object always-bump-effect-counter? drop f ;
         notify-observers
     ] if-bootstrapping ;
 
-TUPLE: nesting-observer new-words ;
+TUPLE: nesting-observer { new-words hash-set } ;
 
 M: nesting-observer definitions-changed
     [ members ] dip new-words>> [ delete ] curry each ;