Steps to replicate the issue (include links if applicable):

• Go to Z35585 to try out the function
• Try adding a row to the argument matrix

What happens?:

There's a greyed-out type field. If you open the menu for Item 1, it's described as a Literal (there's normally a label, like Literal Rational number or Literal Typed list as it should be in this case). If you check the outer list's element type, it's [Object object].
(Compare Z20080.)

What should have happened instead?:

A nested list can be made. The interface does support nested lists, you just have to create the outer list yourself, which this case isn't an option so you have to replace the outer list with a call to echo.

Software version (on Special:Version page; skip for WMF-hosted wikis like Wikipedia):

WF

Other information (browser name/version, screenshots, etc.):

FF 153

Probably related to T417266.