Adding Gaussian Blur to a child layer shows a weird line

[sachkanade]

I'm not sure if I'm describing the issue properly, but this is what I'm doing..

• I add a curves layer and make the image darker
• Then I add a Donut tool and make a donut selection
• I make the donut layer a child layer of the curves layer
• I select the Donut child layer
• Click Effects tab and add Gaussian Blur

Doing so shows some lines on the picture.  I'm trying to get the vignette effect on a certain part of the picture, and don't want to use the vignette tool.  Is this a known issue? Is there a work around for this issue?  Thanks in advance for any help someone can provide. 

[firstdefence]

You need to use the doughnut as a mask. and it would be better to use a live filter so you have flexible control.

1. Add curve layer to make darker
2. Add donut shape
3. Right click donut shape and select mask to below
4. Add Live Gaussian Blur Filter and drag to donut mask like in the image below.
   
5. Layer stack should look like this, you can edit filters and adjustments to suit.