When I use solve in legendary-7, The answer was undefined

[ZenoLeee]

link: https://www.duolingo.cn/lesson/unit/5/legendary/7

[anonymoushackerIV]

Hi, which course from which language did you encounter this issue?

[ZenoLeee]

Hi, which course from which language did you encounter this issue?

The last promotion legend course in the fifth part of the first stage of English

After I completed it manually last night, the problem disappeared when I used "solve".