diff --git a/pkgs/build-support/dotnet/nuget-to-nix/nuget-to-nix.sh b/pkgs/build-support/dotnet/nuget-to-nix/nuget-to-nix.sh
index d3796f88e449..c8db14a4d918 100755
--- a/pkgs/build-support/dotnet/nuget-to-nix/nuget-to-nix.sh
+++ b/pkgs/build-support/dotnet/nuget-to-nix/nuget-to-nix.sh
@@ -24,7 +24,7 @@ while read pkg_spec; do
   pkg_sha256="$(nix-hash --type sha256 --flat --base32 "$(dirname "$pkg_spec")"/*.nupkg)"
 
   pkg_src="$(jq --raw-output '.source' "$(dirname "$pkg_spec")/.nupkg.metadata")"
-  if [[ $pkg_src != https://api.nuget.org/* ]]; then
+  if [[ $pkg_src != https://api.nuget.org/* ]] && [[ ! -d $pkg_src ]]; then
     pkg_source_url="${nuget_sources_cache[$pkg_src]:=$(curl -n --fail "$pkg_src" | jq --raw-output '.resources[] | select(."@type" == "PackageBaseAddress/3.0.0")."@id"')}"
     pkg_url="$pkg_source_url${pkg_name,,}/${pkg_version,,}/${pkg_name,,}.${pkg_version,,}.nupkg"
     echo "  (fetchNuGet { pname = \"$pkg_name\"; version = \"$pkg_version\"; sha256 = \"$pkg_sha256\"; url = \"$pkg_url\"; })" >> ${tmpfile}