diff --git a/basis/random/mersenne-twister/mersenne-twister.factor b/basis/random/mersenne-twister/mersenne-twister.factor
index 712883e4b8..c31d338fac 100644
--- a/basis/random/mersenne-twister/mersenne-twister.factor
+++ b/basis/random/mersenne-twister/mersenne-twister.factor
@@ -16,7 +16,7 @@ TUPLE: mersenne-twister seq i ;
 : mt-a HEX: 9908b0df ; inline
 
 : calculate-y ( n seq -- y )
-    [ nth 32 mask-bit ]
+    [ nth 31 mask-bit ]
     [ [ 1+ ] [ nth ] bi* 31 bits ] 2bi bitor ; inline
 
 : (mt-generate) ( n seq -- next-mt )