Why "capture.QueryFrame()" return null ?

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C#

VB

I have check camera connect to pc very good.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using OpenCVDotNet;

namespace WindowsApplication6
{
    public partial class Form1 : Form
    {
        private CVCapture capture;
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            capture = new CVCapture(0);// when  debug i see signal light camera
        }

        private void timer1_Tick(object sender, EventArgs e)
        {
            using (CVImage nextFrame = capture.QueryFrame())// why nextFrame set null? 

                {
                    if (nextFrame == null) capture.Restart();
                    selectPictureBox1.Image = nextFrame.ToBitmap();
	            }
        }

        private void button1_Click(object sender, EventArgs e)
        {
            timer1.Enabled = !(timer1.Enabled);
        }
    }
}

// I click mouse on "My Computer" icon. Then I double click on "USB Video Device #2". I see my picture. Its mean my camera has work.
My problem why nextFrame set null value.???
please help me! Thanks!