diff --git a/package.json b/package.json
index 1af3eb586365..0de01b6e2c2d 100644
--- a/package.json
+++ b/package.json
@@ -70,7 +70,7 @@
     "ultradumbBenchmark": "./lighthouse-core/scripts/benchmark.js",
     "minify-latest-run": "./lighthouse-core/scripts/lantern/minify-trace.js ./latest-run/defaultPass.trace.json ./latest-run/defaultPass.trace.min.json && ./lighthouse-core/scripts/lantern/minify-devtoolslog.js ./latest-run/defaultPass.devtoolslog.json ./latest-run/defaultPass.devtoolslog.min.json",
     "compile-proto": "protoc --python_out=./ ./proto/lighthouse-result.proto && mv ./proto/*_pb2.py ./proto/scripts || (echo \"❌ Install protobuf ≥ 3.7.1 to compile the proto file.\" && false)",
-    "build-proto-roundtrip": "mkdir -p .tmp && cross-env PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION_VERSION=2 python proto/scripts/json_roundtrip_via_proto.py",
+    "build-proto-roundtrip": "mkdir -p ./tmp && cd proto/scripts && PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=cpp PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION_VERSION=2 python json_roundtrip_via_proto.py",
     "static-server": "node lighthouse-cli/test/fixtures/static-server.js"
   },
   "devDependencies": {