cannot update model: it has no pk in windows phone 8.1 c#

I have a model class shown below.

public class Section
{
    [PrimaryKey]
    public int section_id { get; set; }
    public string name { get; set; }
    public string url { get; set; }
    public System.DateTime syncTime { get; set; }
}

When i try to update I get the error message as Cannot update Section: it has no PK

Below is my update Code

async public Task<bool> UpdateSection_SyncTime(int section_Id)
{
    try
    {
        var sections = await db.Table<Section>().ToListAsync();

        foreach (var section in sections)
        {
            if(section.section_id == section_Id)
            {
                section.syncTime = DateTime.Now;    // Update the Section Sync Time

                if (await db.UpdateAsync(section) >= 1)
                    return true;
            }
        }
    }
    catch { }
    return false;
}

The exception is thrown in SQLite.cs file with the error.