heart rate query returning all data points

I'm developing an iPhone app that retrieves data from the last recorded workout. I'm able to retrieve distance, calories, and duration no problem (lines 01-19 below). However, despite using a predicate with a start and end date (line 22), every single heart rate ever recorded is being returned rather than just those for the given workout. I guess the predicate isn't working, but I can't figure out why.

Any suggestions?

let predicate =  HKQuery.predicateForWorkoutsWithWorkoutActivityType(HKWorkoutActivityType.Running)
let sortDescriptor = NSSortDescriptor(key:HKSampleSortIdentifierStartDate, ascending: false)
let sampleQuery = HKSampleQuery(sampleType: HKWorkoutType.workoutType(), predicate: predicate, limit: 0, sortDescriptors: [sortDescriptor])
     { (sampleQuery, results, error) -> Void in
               if results == nil {
                    print("There was an error running the query: \(error)")
               } else {
                    self.workout = results as! [HKWorkout]
                   
                    let distance = self.workout.first?.totalDistance
                    let duration = self.workout.first?.duration
                    let energy = self.workout.first?.totalEnergyBurned
                    print("Distance: \(distance); Duration: \(duration); Energy: \(energy)")
                    self.dataLabel.text = "Distance: \(distance); Duration: \(duration); Energy: \(energy)"
               }
     }
       
self.healthStore.executeQuery(sampleQuery)
       
let heartRateType = HKQuantityType.quantityTypeForIdentifier(HKQuantityTypeIdentifierHeartRate)!
let predicateHR = HKQuery.predicateForSamplesWithStartDate(self.workout.first?.startDate, endDate: self.workout.first?.endDate, options: HKQueryOptions.None)
let sampleQueryHR = HKSampleQuery(sampleType: heartRateType, predicate: predicateHR, limit: 0, sortDescriptors: nil)
     { (sampleQueryHR, result, error) -> Void in
               if result == nil {
                    print("There was an error running the query: \(error)")
               } else {
                    for quantitySample in result! {
                        let quantity = (quantitySample as! HKQuantitySample).quantity
                        let quantityString = "\(quantity)"
                        let quantityStringNumber = quantityString.stringByReplacingOccurrencesOfString(" count/s", withString: "")
                       
                        if let doubleVersion = Double(quantityStringNumber) {
                            let doubleVersion60 = doubleVersion * 60
                            self.heartRateArray.append(doubleVersion60)
                        } else {
                            print("String can't be converted")
                        }
                }
                   
                  print(self.heartRateArray)
                   
                }
            }
       
self.healthStore.executeQuery(sampleQueryHR)