While trying to use the AWSTransferutility to upload photos with Swift, I get this error. Ambiguous reference to member async(execute:) I searched similar questions but didn't find solution.

func uploadS3Background (dict: NSMutableDictionary) {

    let  transferUtility = AWSS3TransferUtility.default()
    let expression = AWSS3TransferUtilityUploadExpression()
    
    
    let completionHandler = { (task, error) -> Void in
        
        DispatchQueue.main.async(execute: {
            
        })
    }
    
    transferUtility.uploadFile(URL(fileURLWithPath: dict.object(forKey: "url") as! String),
    bucket: "bucketname/images",
    key: (dict.object(forKey: "filename") as! String),
    contentType: "image/png",
    expression: expression,
    completionHandler).continueWith { (task) -> AnyObject! in
        
        if let error = task.error
        {
            print("Error: \(error.localizedDescription)")
        }
            
        if let _ = task.result
        {
            // Do something with uploadTask.
        }
        
        return nil;
    }
}

Outside of a completion block I don't get the error for dispatching on the main queue.