Simple Calculated Question: unable to change / delete wildcards

If I create a simple calculated question like this:

• Answer 1:
  • formula:

    sqrt((4*{w}*{x})/({y}*{z}))

  • grade: 100%

Save (page reloads).
And then click on the "Find the wildcards..." button. (page reloads)
And then click on the "Generate" 1 new set(s) of wild card(s) values button. (page reloads)
And then click on the "Save changes" button.

The question is created fine.

Now if I try to edit the question, changing the formula in any of the following ways:

• remove one of the wildcards, e.g. change the formula to

  sqrt((4*{x})/({y}*{z}))

• change one of the wildcards representation, e.g. change the formula to

  sqrt((4*{w}*{x})/({y}*{a}))

  (z replaced with a)

And then click on the "Find the wildcards..." button. (page reloads)
And then click on the "Generate" 1 new set(s) of wild card(s) values button. (page reloads)
And then click on the "Save changes" button.
Then I see the message highlighted in pink:

You must add at least one set of wild card(s) values before you can save this question.

I also tried removing the formula from answer 1 and setting it's grade to "None", and adding another answer (answer 2), and setting the answer 2 formula to what I want my new formula to be, and setting it's grade to 100%. Then clicking on "Find the wildcards", "Generate", and "Save changes". This also fails to save, showing the message "You must add at least one set of wild card(s) values before you can save this question."