diff --git a/extra/math/similarity/similarity.factor b/extra/math/similarity/similarity.factor
index 0ab7e04264..cdcbc7e469 100644
--- a/extra/math/similarity/similarity.factor
+++ b/extra/math/similarity/similarity.factor
@@ -2,7 +2,7 @@
 ! See http://factorcode.org/license.txt for BSD license
 
 USING: kernel math math.functions math.statistics math.vectors
-sequences sequences.extras ;
+sequences sequences.extras sets ;
 
 IN: math.similarity