diff --git a/basis/io/files/listing/listing-tests.factor b/basis/io/files/listing/listing-tests.factor
index a2347c8db9..8c2dc28559 100644
--- a/basis/io/files/listing/listing-tests.factor
+++ b/basis/io/files/listing/listing-tests.factor
@@ -3,4 +3,6 @@
 USING: tools.test io.files.listing strings kernel ;
 IN: io.files.listing.tests
 
+\ directory. must-infer
+
 [ ] [ "" directory. ] unit-test