405 Error from certain links on ImageButton postbackurl

Hi, I have an imageButton inside a datalist and the postbackurl value is databound. On 99% of the links they all open correctly but sometimes a certain URL will fire back  a 405 error. Does anyone know what is causing this or better yet a workaround?
Would OnClientClick be a better option?
Here's the markup:
<asp:ImageButton ID="imgbRightColumn" runat="server"         AlternateText='<%# Eval("AlternateText") %>'         ImageUrl='<%# Eval("pkid", "viewImageRightColumn.aspx?Id={0}") %>'         PostBackUrl= '<%# Eval("NavURL") %>'         ToolTip='<%# Eval("Tooltip") %>'
/>
...