As disussed at T396359#11045569 and T396359#11046824 :

@dr0ptp4kt wrote:
Although the apparent (?) logic issue for an empty set regex match maybe needs a new task filed (or a patch added to this ticket if that's okay), as for the MalformedHeaderError showing up in a stream, I'm wondering what gives? I may be mistaken and need to look at this when I'm fresher, but wanted to pass this along.

@Ottomata wrote:
I'll try to reproduce the MalformedHeaderError in error stream thing.

Feel free to add patch to this task to fix logic.

I'm filing this as a separate task just to handle separately. I think the particular case is very edge-case-y, but would like to ensure it's handled, nonetheless, mainly to avoid surprises later in case we encounter it.