Error when upgrading to node 6.1.0 npm 3.8.9

[ronnross]

I upgraded a project to use node 6.1.0 and npm 3.8.9. Now receiving an error

path.js:7
    throw new TypeError('Path must be a string. Received ' + inspect(path));

I've seen an error like this pop up in other libs that move to node 6.