diff --git a/google-oauth2-jwt.cabal b/google-oauth2-jwt.cabal
--- a/google-oauth2-jwt.cabal
+++ b/google-oauth2-jwt.cabal
@@ -1,5 +1,5 @@
 name:                google-oauth2-jwt
-version:             0.3.0
+version:             0.3.1
 synopsis:            Get a signed JWT for Google Service Accounts
 description:         This library implements the creation of the
                      signed JWT for Google Service Accounts.
@@ -23,7 +23,7 @@
                   , HsOpenSSL           >= 0.11.1.1 && < 0.12
                   , RSA                 >= 2.1.0.3  && < 2.4
                   , text                >= 1.2.2    && < 1.3
-                  , unix-time           >= 0.3.6    && < 0.4
+                  , unix-time           >= 0.3.6    && < 0.5
 
   default-language: Haskell2010
   GHC-options:      -Wall
diff --git a/src/Network/Google/OAuth2/JWT.hs b/src/Network/Google/OAuth2/JWT.hs
--- a/src/Network/Google/OAuth2/JWT.hs
+++ b/src/Network/Google/OAuth2/JWT.hs
@@ -36,8 +36,8 @@
                                              readPrivateKey)
 import           OpenSSL.RSA
 
-data SignedJWT =
-  SignedJWT !B.ByteString
+newtype SignedJWT =
+  SignedJWT B.ByteString
   deriving (Eq)
 
 instance Show SignedJWT where