JavaScript error, cannot expand or collapse ASP.NET TreeView in SharePoint 2013 Online, Sandboxed Solution

I cannot get a TreeView to work in a custom web part I'm trying to create. I've tried dumbing it down as much as possible, eliminating the C# code for the sake of testing, and it's not working.

This is the JavaScript error I get when I'm trying to expand/collapse a node (via Chrome console, IE doesn't work either). The images [+], [-] are also broken. I would post a picture but I am being denied.

Uncaught ReferenceError: TreeView_ToggleNode is not defined

<asp:TreeView ID="tvOrganization" runat="server"><Nodes><asp:TreeNode Text="CEO"><asp:TreeNode Text="Vice President"><asp:TreeNode Text="Director Development"><asp:TreeNode Text="Manager Web Development" NavigateUrl="#"></asp:TreeNode><asp:TreeNode Text="Manager Marketing" NavigateUrl="#"></asp:TreeNode>                                    </asp:TreeNode>    <asp:TreeNode Text="Director Finance"><asp:TreeNode Text="Manager Finance" NavigateUrl="#"></asp:TreeNode><asp:TreeNode Text="Manager Audit" NavigateUrl="#"></asp:TreeNode></asp:TreeNode></asp:TreeNode>                                </asp:TreeNode></Nodes>    </asp:TreeView>