diff --git a/windows12.cabal b/windows12.cabal
index 71933c1..028c606 100644
--- a/windows12.cabal
+++ b/windows12.cabal
@@ -82,7 +82,8 @@ executable windows12
         text >= 1.2.5 && < 1.3,
         parser-combinators >= 1.3.0 && < 1.4,
         prettyprinter >= 1.5.1 && < 1.6,
-        string-conversions >= 0.4.0 && < 0.5
+        string-conversions >= 0.4.0 && < 0.5,
+        mtl >= 2.2.2 && < 2.3,
 
     -- Directories containing source files.
     hs-source-dirs:   src