localStorage.setItem polyfill throws error on keys with spaces

[trein-homeroom]

Type: bug

Details

1. Load the polyfill. I did so by doing a) in chrome, changing my user agent to be IE7, b) I deleted localStorage before the call to load the polyfill (delete window.localStorage)

2. call localStorage.setItem('something with spaces', true);

Result:
polyfill.min.js?features=localStorage:8 Uncaught DOMException: Failed to execute 'setAttribute' on 'Element': 'userdataf f' is not a valid attribute name.
at https://polyfill.io/v3/polyfill.min.js?features=localStorage:8:213
at Array.forEach ()
at n (https://polyfill.io/v3/polyfill.min.js?features=localStorage:8:184)
at t.setItem (https://polyfill.io/v3/polyfill.min.js?features=localStorage:8:809)
at :1:21
(anonymous) @ polyfill.min.js?features=localStorage:8
n @ polyfill.min.js?features=localStorage:8
setItem @ polyfill.min.js?features=localStorage:8
(anonymous) @ VM717:1

As far as I know its not illegal to pass keys with spaces. I'm not clear on why the polyfill is messing around with using DOM elements as a storage of localStorage.