swift core data executeFetchRequest always crash with propertiesToGroupBy

Question

i am having a problem with executeFetchRequest when i use propertiesToGroupBy. I need to fetch dates(saved as string) but similar dates should be avoided, so that i get unique date array. below is the code i used

    var arrDates: NSArray = NSArray()
    let fetchRequestDates = NSFetchRequest(entityName: "Schedule")
    fetchRequestDates.propertiesToGroupBy = ["date"]
    fetchRequestDates.propertiesToFetch = ["date"]
    fetchRequestDates.resultType = .DictionaryResultType
    do {
        let result = try managedContext.executeFetchRequest(fetchRequestDates)
        arrDates = result as NSArray
    } catch {
        NSLog("Failed to fetch dates: \(error)")
    }

the execution doesn't go further

let result = try managedContext.executeFetchRequest(fetchRequestDates)

and causing SIGBART. If I comment

fetchRequestDates.propertiesToGroupBy = ["date"]

it works but will fetch duplicate dates too. what is the work around?