A2S compiler bug resulting in "Invalid VM register index"

[olofson]

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PipeRumble(pP pV=1 pX)
{
	struct {
		fm1		O
		waveshaper	W
		panmix		PM
	}
	+tr (pP - 3)
	O.p 0; O.fb 0
	W.amount 3
	PM.pan pX; PM.vol pV
	set

	!e 0
	for {
		*O.a .95
		O.fb rand e
		O.p (e + rand (e + 1))
		*e .95
		d 10
	}
	...

Audiality 2: Invalid VM register index at line 540, columns 35..37 in "sfx.a2s"
   540: 		O.p (e + rand (e + 1))
                                           ^

Wut...?