Automod won't save and no error message given

There is no red text error message to help debug this. Looking at browser dev tools the error I see is

error
: 
500
message
: 
"Internal Server Error"``` 

... which isn't very helpful.

The rule in question:

    type: any
    author:
      satisfy_any_threshold: true
      link_karma: ">9000"
      comment_karma: ">25000"
      combined_karma: ">45000"
    action:
      set_flair: "Test flair"
      overwrite_flair: true
    priority: 1

Other rules save with no issue. I don't understand what I'm overlooking. Any help would be appreciated, thanks.