Just got this too and figured why it really happens. In case others get by here:
Check the readme.md “Usage”. The lib is available in three version for three difference module loaders. In short: if you load it with the <script> tag then you must use the UMD version. You can find it in /dist/umd. The default (in /dist) is the ESNext (ECMA-Script) which can not be loaded using the script tag.