Flatten array issue

    let GetCategories (entries : Entry array) =
        let categories : seq<string []> = query {
            for entry in entries do
            where (entry.Categories.Length > 0)
            select entry.Categories
        }
        let arr : string [][] = Seq.toArray categories
        let flatArr = Array.collect arr
//- flatArr error ---------------
Error   1   This expression was expected to have type
    'a -> 'b []
but here has type
    string [] []