"Unrecognized Guid Format"

public Int32 getprpcount(string userplan,string plan)
{
    try
    {
        obj.conopen();
        SqlCommand cmd = new SqlCommand("property_by_plan", obj.con);
        cmd.CommandType = CommandType.StoredProcedure;
        cmd.Parameters.Add("@userplan", SqlDbType.UniqueIdentifier).Value = new Guid(userplan);
        cmd.Parameters.Add("@plan", SqlDbType.NVarChar, 50).Value = plan;
        Int32 cnt = Convert.ToInt32(cmd.ExecuteScalar());
        return cnt;
    }
    catch { return 0; }
}