diff --git a/basis/math/combinatorics/combinatorics-tests.factor b/basis/math/combinatorics/combinatorics-tests.factor
index ac0933fdbe..4e61f27f0d 100644
--- a/basis/math/combinatorics/combinatorics-tests.factor
+++ b/basis/math/combinatorics/combinatorics-tests.factor
@@ -1,6 +1,7 @@
 USING: kernel math math.combinatorics math.combinatorics.private tools.test sequences ;
 IN: math.combinatorics.tests
 
+[ 1 ] [ -1 factorial ] unit-test ! required by other math.combinatorics words
 [ 1 ] [ 0 factorial ] unit-test
 [ 1 ] [ 1 factorial ] unit-test
 [ 3628800 ] [ 10 factorial ] unit-test
diff --git a/basis/math/combinatorics/combinatorics.factor b/basis/math/combinatorics/combinatorics.factor
index 4f4a20d478..c9bf5c6bf5 100644
--- a/basis/math/combinatorics/combinatorics.factor
+++ b/basis/math/combinatorics/combinatorics.factor
@@ -18,7 +18,7 @@ IN: math.combinatorics
 PRIVATE>
 
 : factorial ( n -- n! )
-    iota 1 [ 1 + * ] reduce ;
+    dup 1 > [ [1,b] product ] [ drop 1 ] if ;
 
 : nPk ( n k -- nPk )
     2dup possible? [ dupd - [a,b) product ] [ 2drop 0 ] if ;