diff --git a/hailgun.cabal b/hailgun.cabal
--- a/hailgun.cabal
+++ b/hailgun.cabal
@@ -10,7 +10,7 @@
 -- PVP summary:      +-+------- breaking API changes
 --                   | | +----- non-breaking API additions
 --                   | | | +--- code changes with no API change
-version:             0.4.0.1
+version:             0.4.0.2
 
 -- A short (one-line) description of the package.
 synopsis:            Mailgun REST api interface for Haskell.
@@ -73,7 +73,7 @@
                         , bytestring         >= 0.10.4 && <= 0.11
                         , aeson              == 0.8.*
                         , text               == 1.2.*
-                        , transformers       == 0.4.*
+                        , transformers       >= 0.3 && <0.5
                         , http-client        == 0.4.*
                         , http-client-tls    == 0.2.*
                         , email-validate     == 2.1.*