throw an error if the val is invalid

[adoyle-h]

Give an error response to developers immediately, when calling ms(null) or ms(undefined) or other invalid things.
It can help developers avoid mistakes, and less debug.

[leo]

Please rebase with master! 😊

[adoyle-h]

@leo I have installed the 71616a6 version. It still dose not work.

[leo]

If you're not familiar with how this works: It basically means that you're syncing the branch on which you've added the changes (within our fork of this repo) with the master branch of this repo.

This technique ensures that no conflicts will occur once we decide to merge the PR.

[adoyle-h]

Sorry, I misread your words. Now it could be merged.