<a-sun-sky
        id="sun"
        material="sunPosition: -0.2 20.1 -5; side: back"
      ></a-sun-sky>

Material Name: 
Material Type: ShaderMaterial

Program Info Log: Fragment shader is not compiled.
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FRAGMENT

ERROR: 0:65: 'luminance' : redefinition
ERROR: 0:206: 'luminance' : variable expected
WARNING: 0:206: '/' : Divide by zero during constant folding
ERROR: 0:211: 'luminance' : variable expected
WARNING: 0:211: '/' : Divide by zero during constant folding
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  60: uniform vec3 sunPosition;
  61: varying vec3 vWorldPosition;
  62: 
  63: vec3 cameraPos = vec3(0., 0., 0.);
  64: 
> 65: uniform float luminance;
  66: uniform float turbidity;
  67: uniform float reileigh;
  68: uniform float mieCoefficient;
  69: uniform float mieDirectionalG;
  70: 
  71: // constants for atmospheric scattering
[...]