Unknown parameter: customRanking

[ChoLianJiet]

I was trying to query using customRanking paramater

final SearchForHits queryHits = SearchForHits(
    indexName: indexName,
    query: searchText,
    hitsPerPage: hitsPerPage,
    page: page,
    filters: filters,
    attributesToRetrieve: attributesToRetrieve,
    customRanking: ranking);

However i get a thrown error
AlgoliaApiException{statusCode: 400, error: {"message":"Unknown parameter: customRanking","status":400}}

The ranking is an empty array [].

When i remove the customRanking parameter, it works totally fine.