N-grams do not sort in ascending/descending order

Hello,

I have got an exampleset that was converted to a wordslist with the process documents from data operator to count the term and document occurrences of my dataset. I also have a duiplicate of this process that generates n-grams. Now, the actual wordlist that has no n-grams can be sorted in ascending/descending order when clicking on the attribute column like "term occurrences"  or "document occurrences", however this does not work on the result that contains the n-grams. Does anyone know why the results with the n-grams cannot be sorted?