diff --git a/Text/Css.hs b/Text/Css.hs
--- a/Text/Css.hs
+++ b/Text/Css.hs
@@ -12,7 +12,7 @@
 import Data.Text.Lazy.Builder (Builder, singleton, toLazyText, fromLazyText, fromString)
 import qualified Data.Text.Lazy as TL
 import qualified Data.Text.Lazy.Builder as TLB
-import Data.Monoid (mconcat, mappend, mempty)
+import Data.Monoid (Monoid, mconcat, mappend, mempty)
 import Data.Text (Text)
 import qualified Data.Text as T
 import Language.Haskell.TH.Syntax
@@ -71,6 +71,9 @@
     { mixinAttrs :: ![Attr Resolved]
     , mixinBlocks :: ![Block Resolved]
     }
+instance Monoid Mixin where
+    mempty = Mixin mempty mempty
+    mappend (Mixin a x) (Mixin b y) = Mixin (a ++ b) (x ++ y)
 
 data TopLevel a where
     TopBlock   :: !(Block a) -> TopLevel a
diff --git a/shakespeare-css.cabal b/shakespeare-css.cabal
--- a/shakespeare-css.cabal
+++ b/shakespeare-css.cabal
@@ -1,5 +1,5 @@
 name:            shakespeare-css
-version:         1.0.5
+version:         1.0.5.1
 license:         MIT
 license-file:    LICENSE
 author:          Michael Snoyman <michael@snoyman.com>