Node TypeError: root path required

I am trying to setup a simple node/express server and I receive the following error

TypeError: root path required

I would like to know how to fix this error. Thanks

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var nodeModulesDir = process.env.NODE_MODULES_DIR,

app.use(express.static(publicDir));
app.use("/node_modules", express.static(nodeModulesDir)); // ERROR AT THIS LINE
app.post("/uploads", onUpload);
app.delete("/uploads/:uuid", onDeleteFile);

via Radex