Hi,

With the update to version 2.2.0 we had a new error being thrown without understanding the issue.

Turns out we were sending an undefined message in our call
md5(undefined)

But the first test in the library
if (message === 'undefined' || message === null) throw new Error('Illegal argument ' + message);
is not properly catching the error as message === 'undefined' won't actually catch undefined variables.

A quick fix would be message === undefined or typeof message === 'undefined' or anything of your convenience.

Thank you for your work,