diff --git a/mandrill.cabal b/mandrill.cabal
--- a/mandrill.cabal
+++ b/mandrill.cabal
@@ -1,5 +1,5 @@
 name:                mandrill
-version:             0.5.5.0
+version:             0.5.6.0
 synopsis:            Library for interfacing with the Mandrill JSON API
 description:         Pure Haskell client for the Mandrill JSON API
 license:             MIT
@@ -41,7 +41,7 @@
     , http-types >= 0.8.0
     , http-client >= 0.3.0
     , http-client-tls >= 0.2.0.0
-    , aeson >= 0.7.0.3 && < 1.6.0.0
+    , aeson >= 0.7.0.3 && < 3
     , microlens-th >= 0.4.0.0
     , blaze-html >= 0.5.0.0
     , QuickCheck >= 2.6 && < 3.0
diff --git a/src/Network/API/Mandrill.hs b/src/Network/API/Mandrill.hs
--- a/src/Network/API/Mandrill.hs
+++ b/src/Network/API/Mandrill.hs
@@ -67,7 +67,7 @@
  , _mmsg_from_email = MandrillEmail <$> f
  , _mmsg_from_name = Nothing
  , _mmsg_to = map newRecipient t
- , _mmsg_headers = H.empty
+ , _mmsg_headers = mempty
  , _mmsg_important = Nothing
  , _mmsg_track_opens = Nothing
  , _mmsg_track_clicks = Nothing
@@ -88,7 +88,7 @@
  , _mmsg_subaccount = Nothing
  , _mmsg_google_analytics_domains = []
  , _mmsg_google_analytics_campaign = Nothing
- , _mmsg_metadata = H.empty
+ , _mmsg_metadata = mempty
  , _mmsg_recipient_metadata = []
  , _mmsg_attachments = []
  , _mmsg_images = []
@@ -122,7 +122,7 @@
 
 --------------------------------------------------------------------------------
 -- | Create a new template message (no HTML) with recipient addresses only.
--- This function is preferred when the template being used has the sender 
+-- This function is preferred when the template being used has the sender
 -- address and subject already configured in the Mandrill server.
 newTemplateMessage' :: [EmailAddress]
                     -- ^ Receivers email
diff --git a/src/Network/API/Mandrill/Types.hs b/src/Network/API/Mandrill/Types.hs
--- a/src/Network/API/Mandrill/Types.hs
+++ b/src/Network/API/Mandrill/Types.hs
@@ -31,7 +31,6 @@
 import           Data.Foldable
 import           Data.Traversable
 #endif
-import qualified Data.HashMap.Strict           as H
 import           Data.Monoid
 import qualified Data.Text                     as T
 import qualified Data.Text.Encoding            as TL
@@ -340,7 +339,7 @@
                               <*> pure (MandrillEmail <$> emailAddress "sender@example.com")
                               <*> pure Nothing
                               <*> resize 2 arbitrary
-                              <*> pure H.empty
+                              <*> pure mempty
                               <*> pure Nothing
                               <*> pure Nothing
                               <*> pure Nothing
@@ -361,7 +360,7 @@
                               <*> pure Nothing
                               <*> pure []
                               <*> pure Nothing
-                              <*> pure H.empty
+                              <*> pure mempty
                               <*> pure []
                               <*> pure []
                               <*> pure []