Multitexturing and alpha, round 2 (help please)

I've been wrestling with this for a while now... I'm attempting to draw on a background texture with an overlay texture. Everything works fine until I set EntityAlpha to less than 1 - the background texture disappears entirely! Even an alpha setting of .99 causes this. Here is some code which demonstrates the problem:

Graphics3D 800, 600, 0, 2
camera = CreateCamera()
PositionEntity camera, 0, 0, -5
backdrop = CreateCube()
ScaleEntity(backdrop, 10, 10, 1)
PositionEntity(backdrop, 0, 0, 10)

Const textureblend_ALPHA = 1, textureblend_MULTIPLY = 2, textureblend_ADD = 3
Const loadtexture_COLOUR = 1, loadtexture_ALPHA = 2, loadtexture_MASKED = 4
Const BLUE = 0, GREEN = 8, ALPHA = 24

t1 = CreateTexture(64, 64, loadtexture_COLOUR+loadtexture_ALPHA)
TextureBlend(t1, textureblend_ALPHA)
For x=0 To 63:For y=0 To 63
	WritePixel(x,y,(Rand(255) Shl BLUE) + (255 Shl ALPHA),TextureBuffer(t1))
Next:Next

t2 = CreateTexture(64, 64, loadtexture_COLOUR+loadtexture_ALPHA)
TextureBlend(t2, textureblend_ALPHA)
For x=0 To 63:For y=0 To 63
	WritePixel(x,y,0,TextureBuffer(t2))
Next:Next
For x=20 To 45:For y=20 To 45
	WritePixel(x,y,(Rand(255) Shl GREEN) + (255 Shl ALPHA),TextureBuffer(t2))
Next:Next

c = CreateCube()
RotateEntity(c, -15, -20, 0)
EntityTexture(c, t1, 0, 0)
EntityTexture(c, t2, 0, 1)

While Not KeyHit(1)
	EntityAlpha(c, 1)
	RenderWorld : Flip
	Delay 200
	EntityAlpha(c, .5)
	RenderWorld : Flip
	Delay 200
Wend
End

The cube should be blue noise with a green square of noise in the middle of it, and alternate between being opaque and 50% translucent. On the three computers I've tested this on, the blue background texture disappears entirely when EntityAlpha drops below 1.