Best way for iRule to adapt to offline Virtual?

We have some test backend servers that are regularly offline for prolonged periods of time. However, automated monitoring traffic continues to come in, and causes TCL Errors like this:

01220001:3: TCL error:  - Operation not supported (line 😎     invoked from within "HTTP::header sanitize $preserve_headers"

This is normally caused by attempting to manipulate HTTP headers after an HTTP::redirect, but there is no redirect in this case... I think the backend being offline has the same effect in blocking HTTP:: manipulations.

What's the normal way of dealing with this? Is there a call you can make to check if the backend is offline so that iRules can stop processing before trying to make these changes? Is there a way to tell the F5 not to invoke iRules when the backend server is offline?

Any pointers appreciated.