diff --git a/pkgs/development/haskell-modules/generic-builder.nix b/pkgs/development/haskell-modules/generic-builder.nix
index 2a71e7e92d1..2724ff5d622 100644
--- a/pkgs/development/haskell-modules/generic-builder.nix
+++ b/pkgs/development/haskell-modules/generic-builder.nix
@@ -377,6 +377,10 @@ stdenv.mkDerivation ({
     runHook postHaddock
   '';
 
+  # The scary sed expression handles two cases in v2.5 Cabal's package configs:
+  # 1. 'id:    short-name-0.0.1-9yvw8HF06tiAXuxm5U8KjO'
+  # 2. 'id:\n
+  #         very-long-descriptive-useful-name-0.0.1-9yvw8HF06tiAXuxm5U8KjO'
   installPhase = ''
     runHook preInstall
 
@@ -391,7 +395,7 @@ stdenv.mkDerivation ({
         rmdir "$packageConfFile"
       fi
       for packageConfFile in "$packageConfDir/"*; do
-        local pkgId=$( ${gnused}/bin/sed -n -e 's|^id:[ ]\+||p' $packageConfFile )
+        local pkgId=$( ${gnused}/bin/sed -n -e ':a' -e '/^id:$/N; s/id:\n[ ]*\([^\n]*\).*$/\1/p; s/id:[ ]*\([^\n]*\)$/\1/p; ta' $packageConfFile )
         mv $packageConfFile $packageConfDir/$pkgId.conf
       done