Error: TabContainer1_ActiveTabChanged is undefined

I added OnClientActiveTabChanged="TabContainer1_ActiveTabChanged" to a TabContainer control on an ASPX page.

The code is in the code behind:

public void TabContainer1_ActiveTabChanged(object sender, EventArgs e)
        {
            this.ViewState["ActiveTabIndex"] = TabContainer1.ActiveTabIndex;
        }

When I run the page, the screen displays with the whole tab container thing missing, and an error on the page.

The error says TabContainer1_ActiveTabChanged is undefined.

What am I doing wrong?