I see what you mean - although the popup's use of the words "...to cast a vote" aren't necessarily wrong, as (like you mentioned) it doesn't specify *which type* of vote. I can only assume that when building StackExchange, the developers simply used the word "vote" (rather than "upvote" or "downvote") so that the code behind the popup could be reused for both upvote and downvote popups - if this was the case, it wouldn't be considered a *bug* (see [Gartner's definition][1] of "bug"). Therefore, I'm not sure it would be correct to refer to the prompt as "[erronous][2]".

Also, do keep in mind that when casting a downvote, 1 reputation will be deducted from your total reputation (to prevent malicous or mass-downvoting from users across SE websites).


  [1]: https://www.gartner.com/en/information-technology/glossary/bug
  [2]: https://www.merriam-webster.com/dictionary/erroneous