Not able to get value of dropdownlist.selectedvalue on SelectedIndexChanged - asp.net

Question

After much much research I cannot find the same problem anywhere, under my circumstances. I've enabled viewstate at the page, control, and individual dropdown levels. Page_Load only loads the dropdown if PostBack true according to code.
The problem is that on postback (triggered by the DropDownList or any control), "Empty Parameter" shows in my debugging label, which tells me it's not grabbing the SelectedValue from the dropdown in its selectedIndexChanged method. Also, my dropdowns reset to their initial value before the PageLoad "PopulateFilterDropdowns" method.
Any ideas? I've been searching here, google, anywhere and everywhere I can go. Everybody's problem is usually a lack of "IsPostback" or EnableViewState="true", but no dice on my end. I will mention this is a ascx UserControl, inside of a page with a MasterPage. But I believe the code of each of those is irrelevant to this problem, and quite large to post here.
Help!?
Dropdown:
<asp:DropDownList ID="ddlTopics" runat="server" EnableViewState="true" AutoPostBack="true" OnSelectedIndexChanged="ddlTopics_SelectedIndexChanged">
<asp:ListItem Text="Topic" Value=""></asp:ListItem>
</asp:DropDownList>
Page_Load:
protected void Page_Load(object sender, EventArgs e)
{
if (!Page.IsPostBack)
{
PopulateFilterDropdowns();
}
}
Following methods:
public void PopulateFilterDropdowns()
{
PopulateTopicDropdown();
PopulateAssetTypeDropdown();
PopulateLevelDropdown();
}
public void PopulateTopicDropdown()
{
using (SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["XXX"].ConnectionString))
{
conn.Open();
using (SqlCommand cmd = new SqlCommand("StoredProc", conn))
{
cmd.CommandType = CommandType.StoredProcedure;
using (SqlDataReader rdr = cmd.ExecuteReader())
{
while (rdr.Read())
{
//ListItem itm = new ListItem();
//itm.Value = rdr[0].ToString();
//itm.Text = rdr[0].ToString();
ddlTopics.Items.Add(new ListItem(rdr[0].ToString(), rdr[0].ToString()));
}
}
}
}
}
Problem zone:
protected void ddlTopics_SelectedIndexChanged(object sender, EventArgs e)
{
DropDownList ddlTopics1 = (DropDownList)sender;
string topic = ddlTopics1.SelectedValue;
if (String.IsNullOrEmpty(topic))
lblDebug.Text = "Empty Parameter";
else
lblDebug.Text = topic;
//PopulateRecordsByTopic(topic);
}
public void PopulateRecordsByTopic(string topic)
{
//Query and Binding of repeater on page
}