SqlDataSource with DropDownList Control Parameter doesn't read value

I have a drop down list that bound to a SqlDataSource.

I have another drop down list that's bound to a different SqlDataSource.

The second SqlDataSource has the first drop down as a Control Parameter.

I'm trying to do this...

<asp:SqlDataSource ID="sqlDataSource2" runat="server"
    ConnectionString="<%$ ConnectionStrings:ConnectionString %>"
    SelectCommand="SELECT * FROM Test WHERE Param = @param;"
    CancelSelectOnNullParameter="true">
    <SelectParameters>
        <asp:ControlParameter ControlID="dropDown1" Name="param"
            PropertyName="SelectedValue"
            ConvertEmptyStringToNull="true" />
    </SelectParameters>
</asp:SqlDataSource>

dropDown1.SelectedValue = "someValue"
dropDown2.DataBind()

but I don't get any results. However, if I set the second SqlDataSource's Control Parameter to a text box, it works. For example, this works:

<asp:ControlParameter ControlID="txt" Name="param"
    PropertyName="Text"
    ConvertEmptyStringToNull="true" />

txt.Text = "someValue"
dropDown2.DataBind()

Any ideas why this is?