diff --git a/pkgs/development/haskell-modules/with-packages-wrapper.nix b/pkgs/development/haskell-modules/with-packages-wrapper.nix
index ac887a3f60e..be708e7da57 100644
--- a/pkgs/development/haskell-modules/with-packages-wrapper.nix
+++ b/pkgs/development/haskell-modules/with-packages-wrapper.nix
@@ -51,7 +51,7 @@ buildEnv {
   postBuild = ''
     . ${makeWrapper}/nix-support/setup-hook
 
-    ${optionalString ghc.isGhcjs ''
+    ${optionalString (ghc.isGhcjs or false) ''
     cp -r "${ghc}/${ghc.libDir}/"* ${libDir}/
     ''}