“System.NullReferenceException”类型的异常在 ls1.dll

UserModel usermodel = new UserModel();
if ( Request["xingming"].Trim() != "输入姓名")
{
usermodel.name = Request["xingming"].Trim();
}
if ( Request["shenfenzhen"].Trim() != "请填写真实身份证避免被纳入防沉迷系统")
{
usermodel.sfz = Request["shenfenzhen"].Trim();
}
if (Request["zhanghao"].Trim() != "请输入邮箱地址")
{
usermodel.username = Request["zhanghao"].Trim();
}
if (!string.IsNullOrEmpty(Request["mima"].Trim()))
{
usermodel.pwd = Request["mima"].Trim();
}
if ( Request["wenti"].Trim() != "选择一安全提问问题")
{
usermodel.wenti = Request["wenti"].Trim();
}
if ( Request["daan"].Trim()!="答案")
{
usermodel.daan = Request["daan"].Trim();
}
if ( Request["dianhua"].Trim() != "输入电话号码（含区号）")
{
usermodel.phone = Request["dianhua"].Trim();
}
int count = BL_User.user_insert(usermodel);
if (count >= 1)
{
Response.Write("alert(&#39;注册成功&#39;)<script>&quot;);<br> }</p>

字典里的某个Key没有值

request中访问的对象为空，仔细调试一下