Unexpected Error

Are you calling any sub rule within the rule you are calling this flatten function?

We need to see your code.

if(
  isnull(ri!array),
  true,
  length(
    a!flatten(ri!array)
  ) = 0
)

Do you try to create some null-checking expression? If yes, try isNullOrEmpty() or my function I posted about here: https://appian.rocks/2022/10/17/the-magic-of-null-checking

In your implementation, the problem is that the length() function is not happy with null values. So you would have to filter them out before, or use count().

It occured only once and now it is not occurring with null values as well. I need to find why it occured only that time.

Yeah, I wasn't able to generate an error against the expression you ran even for a variety of null values, so it may have been something else.  Occasionally (due to outside factors) it's possible, if rare, to get an unrelated quirk.

I received this error in Java. In my case, it means that you are iterating over an array, and internally, your variable is converted into a different, local variable (e.g., local1 or local3). Check the elements of your array for null values.

I did check for null values, it does not occur any more.