diff -ru orig/System/Log/FastLogger/Logger.hs new/System/Log/FastLogger/Logger.hs
--- orig/System/Log/FastLogger/Logger.hs	2013-12-24 08:14:25.325658733 +0200
+++ new/System/Log/FastLogger/Logger.hs	2013-12-24 08:14:24.000000000 +0200
@@ -19,6 +19,15 @@
 
 data Logger = Logger (MVar Buffer) !BufSize (IORef LogStr)
 
+#if !MIN_VERSION_base(4, 6, 0)
+atomicModifyIORef' :: IORef a -> (a -> (a,b)) -> IO b
+atomicModifyIORef' ref f = do
+    b <- atomicModifyIORef ref
+            (\x -> let (a, b) = f x
+                    in (a, a `seq` b))
+    b `seq` return b
+#endif
+
 newLogger :: BufSize -> IO Logger
 newLogger size = do
     buf <- getBuffer size