Updating Cast not showing order field

Hello.

I have tried to change role names and correct "as" property in cast of title https://www.imdb.com/title/tt0094758/reference/ but I have found that after checking I got warning:

• Removing order number
  

  You are removing the order number from this entry. Please provide a replacement order number. If you are positive that this actor/actress was billed in alphabetical order, check the box to continue

Order number has shown neither in Cast and Crew edit window nor in classic editor.

This happens on title Krakatit https://www.imdb.com/title/tt0039539/reference/ too.

There is not way to edit credit information without removing order number.