Hello, you are running on Windows, so you may have to avoid harmony-proxies. In your package.json change in scripts / test : cross-env NODE_ENV=test PORT=8080 MONGO_URL=mongodb://localhost:27017/mern-test node_modules/.bin/nyc node --harmony-proxies node_modules/.bin/ava
with : cross-env NODE_ENV=test PORT=8080 MONGO_URL=mongodb://localhost:27017 node_modules/.bin/nyc node_modules/.bin/ava --serial