diff --git a/pkgs/build-support/rust/default.nix b/pkgs/build-support/rust/default.nix
index 2575c7f3107..e027853255c 100644
--- a/pkgs/build-support/rust/default.nix
+++ b/pkgs/build-support/rust/default.nix
@@ -77,6 +77,7 @@ let
     '';
 
   targetIsJSON = stdenv.lib.hasSuffix ".json" target;
+  useSysroot = targetIsJSON && !__internal_dontAddSysroot;
 
   # see https://github.com/rust-lang/cargo/blob/964a16a28e234a3d397b2a7031d4ab4a428b1391/src/cargo/core/compiler/compile_kind.rs#L151-L168
   # the "${}" is needed to transform the path into a /nix/store path before baseNameOf
@@ -202,8 +203,8 @@ stdenv.mkDerivation ((removeAttrs args ["depsExtraArgs"]) // {
       "CXX_${rust.toRustTarget stdenv.buildPlatform}"="${cxxForBuild}" \
       "CC_${rust.toRustTarget stdenv.hostPlatform}"="${ccForHost}" \
       "CXX_${rust.toRustTarget stdenv.hostPlatform}"="${cxxForHost}" \
-      ${stdenv.lib.optionalString
-          (targetIsJSON && !__internal_dontAddSysroot) "RUSTFLAGS=\"--sysroot ${sysroot} $RUSTFLAGS\" "
+      ${stdenv.lib.optionalString useSysroot
+         "RUSTFLAGS=\"--sysroot ${sysroot} $RUSTFLAGS\" "
       }cargo build -j $NIX_BUILD_CORES \
         ${stdenv.lib.optionalString (buildType == "release") "--release"} \
         --target ${target} \