diff --git a/Mail/Hailgun/Internal/Data.hs b/Mail/Hailgun/Internal/Data.hs
--- a/Mail/Hailgun/Internal/Data.hs
+++ b/Mail/Hailgun/Internal/Data.hs
@@ -33,7 +33,7 @@
 #endif
 
 type UnverifiedEmailAddress = B.ByteString -- ^ Represents an email address that is not yet verified.
-type MessageSubject = String -- ^ Represents a message subject.
+type MessageSubject = T.Text -- ^ Represents a message subject.
 
 -- | A generic error message that is returned by the Hailgun library.
 type HailgunErrorMessage = String
diff --git a/Mail/Hailgun/SendEmail.hs b/Mail/Hailgun/SendEmail.hs
--- a/Mail/Hailgun/SendEmail.hs
+++ b/Mail/Hailgun/SendEmail.hs
@@ -8,6 +8,7 @@
 import           Data.Aeson
 import qualified Data.ByteString.Char8                 as BC
 import qualified Data.Text                             as T
+import qualified Data.Text.Encoding                    as T
 import           Mail.Hailgun.Communication
 import           Mail.Hailgun.Errors
 import           Mail.Hailgun.Internal.Data
@@ -41,7 +42,7 @@
 toSimpleEmailParts :: HailgunMessage -> [(BC.ByteString, BC.ByteString)]
 toSimpleEmailParts message =
    [ (BC.pack "from", toByteString . messageFrom $ message)
-   , (BC.pack "subject", BC.pack $ messageSubject message)
+   , (BC.pack "subject", T.encodeUtf8 $ messageSubject message)
    ] ++ to
    ++ cc
    ++ bcc
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:             0.3.0.0
+version:             0.4.0.0
 
 -- A short (one-line) description of the package.
 synopsis:            Mailgun REST api interface for Haskell.