When using a GridView, SelectedIndex is always 0 when using custom select button - asp.net

Question

When clicking the select button in gvSquid2, x gets a valid and correct value. When clicking the select button in gvSquid, I get an ArgumentOutOfRange Exception because gv.SelectedIndex=0. Additionally, gv.SelectedDataKey is null. How do I get the DataKey information? Putting it in a column is not acceptable, as it must be hidden from the user. Hiding the column is useless, because during DataBind(), the value is discarded. I ripped this example from Microsoft's website. What am I missing?
<asp:gridview ID="gvSquid2" DataSourceID="dsComments" AutoGenerateColumns="false" AutoGenerateSelectButton="true" DataKeyNames="ID" OnSelectedIndexChanged="gvSquid_SelectedIndexChanged" runat="server" EnableViewState="false">
<Columns>
<asp:BoundField DataField="Date" ReadOnly="true" HeaderText="Date" SortExpression="Date" />
<asp:BoundField DataField="Comment" ReadOnly="true" HeaderText="Comment" SortExpression="Comment" />
<asp:BoundField DataField="Username" ReadOnly="true" HeaderText="User" SortExpression="Username" />
</Columns>
</asp:gridview>
<asp:gridview ID="gvSquid" DataSourceID="dsComments" AutoGenerateColumns="false" DataKeyNames="ID" OnSelectedIndexChanged="gvSquid_SelectedIndexChanged" runat="server" EnableViewState="false">
<Columns>
<asp:CommandField ButtonType="Image" HeaderText="Select" ShowSelectButton="true" SelectImageUrl="~/includes/RedX.jpg" />
<asp:BoundField DataField="Date" ReadOnly="true" HeaderText="Date" SortExpression="Date" />
<asp:BoundField DataField="Comment" ReadOnly="true" HeaderText="Comment" SortExpression="Comment" />
<asp:BoundField DataField="Username" ReadOnly="true" HeaderText="User" SortExpression="Username" />
</Columns>
</asp:gridview>
protected void gvSquid_SelectedIndexChanged(object sender, EventArgs e) {
string x;
x = gvSquid.DataKeys[gvSquid.SelectedIndex].Value.ToString();
}

Answer 1

Try using the SelectedRow of the grid view to get to the RowIndex, like this:
protected void gvSquid_SelectedIndexChanged(object sender, EventArgs e)
{
string x;
GridViewRow theGridViewRow = gvSquid.SelectedRow;
x = gvSquid.DataKeys[theGridViewRow.RowIndex].Value.ToString();
}

Answer 2

I figured it out. EnableViewState="true" has to be set. Once I changed that, suddenly the DataKeys appeared.
This question helped.