Uncaught TypeError: fs.readdirSync is not a function

[ezmill]

Calling var site = nanocontent.readSiteSync('./content') gives error:

Uncaught TypeError: fs.readdirSync is not a function
    at GlobSync._readdir (sync.js:288)
    at GlobSync._processGlobStar (sync.js:350)
    at GlobSync._process (sync.js:130)
    at new GlobSync (sync.js:4)
    at Function.globSync [as sync] (sync.js:26)
    at Object.readSiteSync (readSiteSync.js:19)
    at Object.readSiteSync (index.js:55)

in the glob module.