diff --git a/pkgs/development/tools/documentation/haddock/2.4.2.nix b/pkgs/development/tools/documentation/haddock/2.4.2.nix
index 226a4fd676e3..30b06ed280a1 100644
--- a/pkgs/development/tools/documentation/haddock/2.4.2.nix
+++ b/pkgs/development/tools/documentation/haddock/2.4.2.nix
@@ -1,4 +1,4 @@
-{ cabal, ghcPaths }:
+{ cabal, alex, ghcPaths }:
 
 cabal.mkDerivation (self: {
   pname = "haddock";
@@ -7,6 +7,7 @@ cabal.mkDerivation (self: {
   isLibrary = true;
   isExecutable = true;
   buildDepends = [ ghcPaths ];
+  buildTools = [ alex ];
   meta = {
     homepage = "http://www.haskell.org/haddock/";
     description = "A documentation-generation tool for Haskell libraries";