diff --git a/pkgs/build-support/rust/fetch-cargo-tarball/default.nix b/pkgs/build-support/rust/fetch-cargo-tarball/default.nix
index 6d6219078bac..7ca306b613bc 100644
--- a/pkgs/build-support/rust/fetch-cargo-tarball/default.nix
+++ b/pkgs/build-support/rust/fetch-cargo-tarball/default.nix
@@ -28,7 +28,11 @@ in
 } @ args:
 
 let hash_ =
-  if args ? hash then { outputHashAlgo = null; outputHash = args.hash; }
+  if args ? hash then
+    {
+      outputHashAlgo = if args.hash == "" then "sha256" else null;
+      outputHash = args.hash;
+    }
   else if args ? sha256 then { outputHashAlgo = "sha256"; outputHash = args.sha256; }
   else throw "fetchCargoTarball requires a hash for ${name}";
 in stdenv.mkDerivation ({