为什么这个form表单验证不成功？

 function checkDelForm() {
        var uids  = -1;
        uids = document.getElementsByName("delUids").length;
        if (uids == -1) {
            alert("请选择要删除的用户");
            return false;
        } else {
            return true;
        }
    }
<form name="userform" action="servlet/DeleteUserServet" method="post"
    onsubmit="return checkDelForm()">
    <table align="center" border="1" width="440px">
        <tr>
            <td colspan="6" align="center">用户资料</td>
        </tr>
        <tr>
            <td align="center">用户姓名</td>
            <td align="center">用户性别</td>
            <td align="center">出生日期</td>
            <td align="center">用户住址</td>
            <td colspan=2 align="center">操作</td>
        </tr>
        <%
            while (rs.next()) {
        %>
        <tr>
            <td><%=rs.getString("uname")%></td>
            <td><%=rs.getString("usex")%></td>
            <td><%=rs.getString("ubirth")%></td>
            <td><%=rs.getString("uaddr")%></td>

            <td><a
                href="UserListDetail.jsp?uid=<%=rs.getInt("uid")%>&&uname=<%=rs.getString("uname")%>&&usex=<%=rs.getString("usex")%>&&ubirth=<%=rs.getString("ubirth")%>&&uaddr=<%=rs.getString("uaddr")%>">编辑</a>
            </td>
            <td><input type="checkbox" name="delUids"
                value="<%=rs.getInt("uid")%>"></td>
        </tr>

        <%
            }
            conn.close();
            rs.close();
            stmt.close();
        %>
        <tr>
            <td colspan="6" align="center"><input type="submit" value="删除"
                style="width: 82px; "> <font color="green"> ${msg} </font>
            </td>
        </tr>
    </table>
</form>