genSalt (async) with no arguments throws callback error instead of returning promise

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I am using bcryptjs in a node/express application and received an error about an undefined argument when trying to run the callback for genSalt although the documentation specifies that since 2.4.0 (which is the version I have) an omitted callback means genSalt will return a promise. I have to specify the number of rounds in order to get a promise back; I cannot use genSalt() with no arguments.