Having trouble getting effects masks to work

I feel like I must be missing something. I can't seem to get images to work as effects masks.

Here's the situation. I have a CG clip. It has a z-depth channel. I want to add in a color correct node, but have the color correction fade into the distance, so I'd like the use the zdepth channel from one image as the effects mask for the color correction node. But it doesn't seem to work. If I create a generator node, like plasma, and plug that into the effects mask it works fine. But when I plug in an image it doesn't work. Nothing happens. The image doesn't change.

I'm using deep EXR images, but it doesn't matter if I pipe them through a "deep to image" node or just have the file loaded as a flat image when I plug it into the effects mask nothing changes. Am I missing something? Please advise. Thanks!

The Color Corrector is a 2D node, so Z information will need to be converted into a 2D matte, like a depth map.

Feed the CG clip with the Z-depth channel into a CopyAux node.
Set to Aux to Color and the Aux Channel to Z.
Enable Remapping (Z to Color).
Click on 'Detect Range', so it 'normalises' the image.
Output of the CopyAux should now look like a depth map.
Feed that into the mask input of the Color corrector, and ensure in the CC settings tab the mask channel is set to luminance.

Interesting. I didn't realize that z-depth pass wasn't treated as a standard 2d image. I'll try this.
Thanks!

The Color Corrector is a 2D node, so Z information will need to be converted into a 2D matte, like a depth map.

Feed the CG clip with the Z-depth channel into a CopyAux node.
Set to Aux to Color and the Aux Channel to Z.
Enable Remapping (Z to Color).
Click on 'Detect Range', so it 'normalises' the image.
Output of the CopyAux should now look like a depth map.
Feed that into the mask input of the Color corrector, and ensure in the CC settings tab the mask channel is set to luminance.

Thank you!! This worked.

Interesting. I didn't realize that z-depth pass wasn't treated as a standard 2d image. I'll try this.
Thanks!

If z depth has direct depth values they might all be bigger than 1.0 but mask expects range of 0.0-1.0 so depth as a mask won't do anything.