Calling capitalize() on undefined returns an unhelpful error message

[MichalBryxi]

• Twiddle here - you need to look into devtools console

The problem

When calling capitalize() on undefined:

import { capitalize } from '@ember/string';

return capitalize(undefined);

One will get a weird error:

Uncaught TypeError: Cannot read property 'replace' of undefined
    at Cache.func (ember.debug.js:41546)
    at Cache.get (ember.debug.js:30881)
    at Object.capitalize (ember.debug.js:41770)
    at ApplicationController.get foo [as foo] (about:srcdoc:35)
    at getPossibleMandatoryProxyValue (ember.debug.js:14419)
    at get (ember.debug.js:14485)
    at ember.debug.js:46909
    at runInAutotrackingTransaction (ember.debug.js:54899)
    at track (ember.debug.js:55486)
    at PropertyReference.value (ember.debug.js:46908)

[MichalBryxi]

Possible solutions:

a) Accept undefined and return empty string. I like that this makes sure that invalid input does not make the consuming app explode, but is that really what should happen? Doesn't that introduce potential bugs into users code?
b) assert() a warning saying that "capitalize helper can't accept undefined as an argument" or something.