diff --git a/src/Data/UnitsOfMeasure/Defs.hs b/src/Data/UnitsOfMeasure/Defs.hs
--- a/src/Data/UnitsOfMeasure/Defs.hs
+++ b/src/Data/UnitsOfMeasure/Defs.hs
@@ -38,7 +38,15 @@
 
 -- Non-SI units accepted for use with them
 -- http://www.bipm.org/en/publications/si-brochure/chapter4.html
-[u| min = 60 s, h = 3600 s, d, ha, l, t, au |]
+-- hectares and liters are unable to be defined at the moment due to
+-- https://github.com/adamgundry/uom-plugin/issues/4
+[u| min = 60 s
+  , h = 3600 s
+  , d = 86400 s
+  , ha
+  , l
+  , t = 1000 kg
+  , au = 149597870700 m |]
 
 -- Some random other units
 [u| ft = 100 % 328 m, in = 0.0254 m, mi = 1609.344 m, mph = mi/h |]
diff --git a/src/Data/UnitsOfMeasure/Internal.hs b/src/Data/UnitsOfMeasure/Internal.hs
--- a/src/Data/UnitsOfMeasure/Internal.hs
+++ b/src/Data/UnitsOfMeasure/Internal.hs
@@ -56,6 +56,8 @@
     , MkUnit
     ) where
 
+import Control.DeepSeq
+import Foreign.Storable
 import GHC.Exts (Constraint)
 import GHC.TypeLits (Symbol, Nat, type (-))
 
@@ -121,6 +123,11 @@
 deriving instance (RealFloat  a, u ~ One) => RealFloat  (Quantity a u)
 deriving instance (RealFrac   a, u ~ One) => RealFrac   (Quantity a u)
 
+-- To enable marshalling into FFI code.
+deriving instance Storable a => Storable (Quantity a u)
+
+-- To enable deriving NFData for data types containing Quantity fields.
+deriving instance NFData a => NFData (Quantity a u)
 
 -- | Extract the underlying value of a quantity
 unQuantity :: Quantity a u -> a
diff --git a/uom-plugin.cabal b/uom-plugin.cabal
--- a/uom-plugin.cabal
+++ b/uom-plugin.cabal
@@ -1,5 +1,5 @@
 name:                uom-plugin
-version:             0.1.0.0
+version:             0.1.1.0
 synopsis:            Units of measure as a GHC typechecker plugin
 category:            Type System
 description:         A prototype typechecker plugin for GHC with support for units of measure
@@ -40,6 +40,7 @@
                        TcPluginExtras
   other-extensions:    TemplateHaskell
   build-depends:       base >=4.7 && <5,
+                       deepseq >=1.3 && <1.5,
                        ghc >= 7.9 && <7.12,
                        ghc-tcplugins-extra >=0.1 && <0.2,
                        template-haskell >=2.9 && <2.12,