diff --git a/System/X509/Unix.hs b/System/X509/Unix.hs
--- a/System/X509/Unix.hs
+++ b/System/X509/Unix.hs
@@ -33,6 +33,7 @@
 
 import Data.Char
 import Data.Maybe (catMaybes)
+import Data.Monoid (mconcat)
 
 defaultSystemPaths :: [FilePath]
 defaultSystemPaths =
diff --git a/x509-system.cabal b/x509-system.cabal
--- a/x509-system.cabal
+++ b/x509-system.cabal
@@ -1,5 +1,5 @@
 Name:                x509-system
-Version:             1.6.2
+Version:             1.6.3
 Description:         System X.509 handling
 License:             BSD3
 License-file:        LICENSE