--- a/lib/prelude.nix
+++ b/lib/prelude.nix
@@ -108,9 +108,24 @@
 
   primitiveFetchers = listFetchers builtinsOverlay true ++ [ "fetchurlBoot" ];
 
+  # Helper to merge args, handling both attrsets and functions (for extendMkDerivation)
+  mergeArgsWithRequired = requiredArgs: args:
+    if isFunction args
+    then final: requiredArgs // args final
+    else requiredArgs // args;
+
+  # Helper to safely intersect args with oldArgs, handling function args
+  safeIntersectArgs = args: oldArgs:
+    if isFunction args
+    then {}  # When args is a function, we can't know the keys at evaluation time
+    else builtins.intersectAttrs args oldArgs;
+
   markFetcher = { type, name, fetcher }:
     let
-      customFetcher = args: markFetcherDrv { inherit type name fetcher args; drv = fetcher (requiredFetcherArgs // args); };
+      customFetcher = args: markFetcherDrv {
+        inherit type name fetcher args;
+        drv = fetcher (mergeArgsWithRequired requiredFetcherArgs args);
+      };
 
       # The required fetcher arguments are assumed to be of type string,
       # because requiring a complex value, e.g. a derivation attrset, is very unlikely,
@@ -132,7 +147,7 @@
               if !(elem origPassthru.__fetcher.name primitiveFetchers) then functionArgs origPassthru.__fetcher
               else throw "Fetcher ${name} is build on top of the primitive fetcher ${origPassthru.__fetcher.name}, which is not supported."
             else {};
-          newArgs = oldArgs // functionArgs fetcher // mapAttrs (_: _: true) (builtins.intersectAttrs args oldArgs);
+          newArgs = oldArgs // functionArgs fetcher // mapAttrs (_: _: true) (safeIntersectArgs args oldArgs);
         in {
           passthru = origPassthru // {
             __fetcher = setFunctionArgs fetcher newArgs // { inherit type name args; drv = drvOverriden; };