bus error in VCL processing

[Mike Perham]

Varnish gives a bus error (2.0.2 on OSX) when I start it up with this VCL:

sub vcl_miss {
    if (!(bereq.url ~ req.http.host)) {
        set bereq.url = "/" req.http.host req.url;
    }
}

I'm trying to add the Host header to the front of the URL before
passing to our S3 backend.  Since relative ESIs use the modified URL,
I need to verify the URL has not already been mangled before.  I
believe the issue is due to the use of req.http.host as the match
substring.  if I replace it with a hardcoded string, the bus error
goes away:

    if (!(bereq.url ~ 'foo')) {

How can I do what I'm trying to do?

mike