Webforms / customErrors not working when using redirectMode="ResponseRewrite"

Hi,
I have the following in my webforms web.config:
<customErrors mode="On" redirectMode="ResponseRewrite" defaultRedirect="~/error.aspx" > <error statusCode="404" redirect="~/404.aspx" /> </customErrors> When I use redirectMode="ResponseRewrite" it appears to stop custom errors from working. Does anyone know why and how to fix? Thanks