ASM Illegal URL garbles Custom Default Response Page

NEVER MIND.

I am an utter, complete, total nincompoop.

I was issuing an HTTP::respond 404 myself in an iRule that I'd forgotten about.

I will attempt to delete this post now.

The ASM block page for Illegal URLs appears to:

• Replace the Response Headers from the Custom (200 -> 404)
• Append the Custom response text to a fixed "Not Found"
• Set the Content-Length to 9 (which is length("Not Found"))
• (The browser will ignore the Custom response text that was appended, due to Content-Length)

Example:

I have an ASM profile where the Default Response Page has been set to Custom and given the following text:

However, if I have

/test/block

configured as an Illegal URL and go to it in a browser, I get the following response (screencap from Burp Suite Proxy Intercept):

Why is the ASM incorrectly combining responses like this? Is there anywhere to configure the 404 (I'm happy to send a 404, but prefer my text) within ASM (I know I can do it with an iRule, but that seems like overkill).

Any help appreciated.