I think that was the issue.
I created a WebGLCubeRenderTarget and used .fromEquirectangularTexture( yourHDR )
and plugged it into your shader, and it works.
https://jsfiddle.net/manthrax/5evb02n8/9/
ps: fwiw MeshStandardMaterial does IBL out of the box:
https://threejs.org/examples/webgl_materials_envmaps_hdr.html