Can't update PR source is required

{"errors": {"source": ["This field is required."]}, "error": "An error occurred saving this pull request. Please check the required fields and try again"}

Uncaught TypeError: Cannot read property 'split' of null
    at t.default (parse-compare-spec.js?b510:4)
    at e (edit.js:261)
    at S (edit.js:287)
    at e.Router.<anonymous> (edit.js:511)
    at e.Router.<anonymous> (index.js:7363)
    at i (director.js?a3c1:517)
    at n (director.js?a3c1:298)
    at e.Router.d.invoke (director.js?a3c1:522)
    at r (director.js?a3c1:480)
    at e.Router.d.dispatch (director.js?a3c1:492)
    at e.Router.handler (director.js?a3c1:186)
    at e.Router.d.init (director.js?a3c1:214)
    at Object.3227 (pullrequests.js:43)
    at t (bootstrap 7c8df45e9723928a37e1?e4b7:52)
    at Object.3935 (pullrequests.js:17678)
    at t (bootstrap 7c8df45e9723928a37e1?e4b7:52)
    at window.webpackJsonp (bootstrap 7c8df45e9723928a37e1?e4b7:23)
    at pullrequests.js:1