diff --git a/pkgs/build-support/rust/default.nix b/pkgs/build-support/rust/default.nix
index 83ef146de5f8..f407ba25bf85 100644
--- a/pkgs/build-support/rust/default.nix
+++ b/pkgs/build-support/rust/default.nix
@@ -4,7 +4,7 @@ let
     inherit stdenv cacert git rust cargo-vendor;
   };
 in
-{ name, cargoSha256 ? null
+{ name, cargoSha256 ? "unset"
 , src ? null
 , srcs ? null
 , sourceRoot ? null
@@ -17,7 +17,7 @@ in
 , cargoVendorDir ? null
 , ... } @ args:
 
-assert cargoVendorDir == null -> cargoSha256 != null;
+assert cargoVendorDir == null -> cargoSha256 != "unset";
 
 let
   lib = stdenv.lib;