diff --git a/nix/build-haskell-package.nix b/nix/build-haskell-package.nix
index 3563395a..0c9bb826 100644
--- a/nix/build-haskell-package.nix
+++ b/nix/build-haskell-package.nix
@@ -34,8 +34,14 @@ lib.pipe root
     (mkNewStorePath "source-${name}")
     (x: log.traceDebug "${name}.mkNewStorePath ${x.outPath}" x)
 
-    (root: self.callCabal2nix name root { })
-    (x: log.traceDebug "${name}.cabal2nixDeriver ${x.cabal2nixDeriver.outPath}" x)
+    (root:
+      if lib.pathExists (lib.concatStringsSep "/" [ root "custom-default.nix" ])
+      then
+        let pkgInfo = self.callPackage (lib.concatStringsSep "/" [ root "custom-default.nix" ]) { };
+        in pkgInfo
+      else
+        let pkgInfo = self.callCabal2nix name root { };
+        in (log.traceDebug "${name}.cabal2nixDeriver ${pkgInfo.cabal2nixDeriver.outPath}" pkgInfo))
 
     # Make sure all files we use are included in the sdist, as a check
     # for release-worthiness.