Custom 404 handler unable to read post variables after installing a passthru wildcard application mapping

Our customer's application uses a custom 404 handler that must read
variables that were passed using POST to the "not found" page. Installing
our ISAPI extension appears to break this functionality. The custom 404
handler is an ASP script. Our ISAPI extension is implemented as a passthru
wildcard application mapping. Environment: IIS 6.0, ASP.NET 2.0.

When the custom 404 handler is called, Request.TotalBytes is a positive
integer. Request.BinaryRead(Request.TotalBytes) throws the following
exception:
---------------------------------------------
object error 'ASP 0101 : 80004005'

Unexpected error

/404.asp, line 12

The function returned |.
-----------------------------------------------

The custom 404 handler works smoothly when opened directly from a HTML form.

Thanks for your help.