"TypeError: Assignment to constant variable" when testing some URLs

[malcomio]

Steps to reproduce

1. install the bookmarklet
2. visit one of the following URLs:
   1. https://www.transizionedigitale.it/
   2. https://www.neso.energy/
3. click the bookmarklet

Expected result

The sniffer should run and report any issues

Actual result

The test does not complete
Errors in console:

Uncaught TypeError: Assignment to constant variable.
    at Object.testNullAltText (HTMLCS.js:24:223570)
    at Object.addNullAltTextResults (HTMLCS.js:24:222007)
    at Object.process (HTMLCS.js:24:221802)
    at m (HTMLCS.js:24:288483)
    at c (HTMLCS.js:24:288090)
    at Object.run (HTMLCS.js:24:287448)
    at HTMLCS.js:24:286284
    at HTMLCS.js:24:286545
    at h (HTMLCS.js:24:289153)
    at HTMLCS.js:24:289131

Originally raised as pa11y/pa11y#709

[malcomio]

Using a debug version, the output is as follows:

HTMLCS.js:2758  Uncaught TypeError: Assignment to constant variable.
    at Object.testNullAltText (HTMLCS.js:2758:18)
    at Object.addNullAltTextResults (HTMLCS.js:2711:27)
    at Object.process (HTMLCS.js:2698:18)
    at _processSniffs (HTMLCS.js:5414:23)
    at _run (HTMLCS.js:5387:17)
    at Object.run (HTMLCS.js:5338:13)
    at HTMLCS.js:5259:24
    at HTMLCS.js:5282:22
    at _registerSniffs (HTMLCS.js:5465:22)
    at HTMLCS.js:5470:13