diff --git a/perf.cabal b/perf.cabal
--- a/perf.cabal
+++ b/perf.cabal
@@ -2,10 +2,10 @@
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: ce01c5e5d57fc57b3bd0e39325c134d3ffb9e7aa21597bbc8458eb894b92991e
+-- hash: 44e8680f9c9810c48b2593a9217d338cc2c64b8884b984279b7337dbaae4eb9d
 
 name:           perf
-version:        0.4.0.0
+version:        0.4.0.1
 synopsis:       Low-level run time measurement.
 description:     A set of tools to accurately measure time performance of Haskell programs.
                 perf aims to be lightweight by having minimal dependencies on standard libraries.
diff --git a/src/Perf.hs b/src/Perf.hs
--- a/src/Perf.hs
+++ b/src/Perf.hs
@@ -61,6 +61,7 @@
 import Control.Monad.Trans.Class (lift)
 import Control.Monad.Trans.State (StateT(..), evalStateT, runStateT, execStateT, get, put )
 import Data.Functor.Identity
+import Data.Monoid ((<>))
 import Perf.Cycle
 import Perf.Measure
 import qualified Data.Map as Map