diff --git a/seqid.cabal b/seqid.cabal
--- a/seqid.cabal
+++ b/seqid.cabal
@@ -1,5 +1,5 @@
 name:                seqid
-version:             0.4.0
+version:             0.4.1
 synopsis:            Sequence ID production and consumption
 description:         Uniquely identify elements in a sequence.
 License:             BSD3
@@ -17,6 +17,7 @@
 library
   exposed-modules: Data.SequenceId
   build-depends: base         >= 4.7 && < 4.9
+               , mtl          >= 2.2 && < 2.3
                , transformers >= 0.4 && < 0.5
 
   hs-source-dirs:      src