Shader BlendMode + Filter = Issue with transparency

Question

When I use custom shader as a blend mode and apply a BlurFilter, black border appears. Without filter it works fine.

Here is a shader code.

kernel NewFilter
<   namespace : "my.test";
    vendor : "Im";
    version : 1;
    description : "Invert Luminance Blend Mode";
>
{
    input image4 background;
    input image4 foreground;
    output pixel4 dst;

    void
    evaluatePixel()
    {
        pixel4 bg = sampleNearest(background, outCoord());
        pixel4 fg = sampleNearest(foreground, outCoord());

        float lum = bg.r * 0.299 + bg.g * 0.587 + bg.b * 0.114;

        dst.rgb = fg.rgb + 1.0 - lum;
        dst.a = fg.a;
    }
}

How I apply shader and filter

[Embed(source = "InvertLuminance.pbj", mimeType = "application/octet-stream")]
    var InvertLuminanceClass:Class;

...

highlighter = new Sprite();
highlighter.mouseEnabled = false;
highlighter.mouseChildren = false;

var shader:Shader = new Shader(new InvertLuminanceClass());

highlighter.blendShader = shader;
highlighter.blendMode = BlendMode.SHADER;
highlighter.filters = [new BlurFilter(1.9, 1.9, BitmapFilterQuality.HIGH)];

addChild(highlighter);

This issue appears with any shader as a blend mode. Can anybody help me with fixing this black pixels?

Thanks!