These lines check if _ is a function, but should really check if it is actually a lodash instance:
https://github.com/lodash/lodash/blob/master/fp/_convertBrowser.js#L15-L17

In our case we still (unfortunately) have some legacy code that needs underscore - we call lodash with noConflict but some code requires underscore to be global. We're working to rip it out completely, but ideally the check here would either be stricter or support some way of opting out of the automatic check (even if it's wrapped in an if (!window.dontAutoFP) or something that we can set before the script loads)