diff --git a/examples/js/SkyShader.js b/examples/js/SkyShader.js
index 9894793530226826db7ba873ce562dd78eaf0048..45068d8febf8f825b2b3ea8fc8c0572bc8f855be 100644
--- a/examples/js/SkyShader.js
+++ b/examples/js/SkyShader.js
@@ -66,6 +66,7 @@ THREE.ShaderLib[ 'sky' ] = {
 
 		"float sunIntensity(float zenithAngleCos)",
 		"{",
+		    	"zenithAngleCos = clamp(zenithAngleCos, -1.0, 1.0);",
 			"return EE * max(0.0, 1.0 - pow(e, -((cutoffAngle - acos(zenithAngleCos))/steepness)));",
 		"}",