vs 两个dropdownlist 联动问题

Question

 我的两个dropdownlist是通过“选择数据源”建立的联动，但是出现“输入字符串的格式不正确”的错误 这是什么原因啊 代码如下：
   <asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True" DataSourceID="SqlDataSource1"
            DataTextField="SurveyName" DataValueField="SurveyName">
        </asp:DropDownList><asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:voteConnectionString3 %>"
            SelectCommand="SELECT * FROM [Survey_Poll]"></asp:SqlDataSource>
        &nbsp;&nbsp;
        <asp:DropDownList ID="DropDownList2" runat="server" AutoPostBack="True" DataSourceID="SqlDataSource2"
            DataTextField="QuestionName" DataValueField="QuestionName">
        </asp:DropDownList><asp:SqlDataSource ID="SqlDataSource2" runat="server" ConnectionString="<%$ ConnectionStrings:voteConnectionString3 %>"
            SelectCommand="SELECT * FROM [Survey_Question] WHERE ([SurveyId] = @SurveyId)">
            <SelectParameters>
                <asp:ControlParameter ControlID="DropDownList1" Name="SurveyId" PropertyName="SelectedValue"
                    Type="Int32" />
            </SelectParameters>
        </asp:SqlDataSource>

Answer 1

DropDownList是不是没有处理空值!或者数值类型不匹配!!