diff --git a/packages/core/src/shaderlib/pbr/ibl_frag_define.glsl b/packages/core/src/shaderlib/pbr/ibl_frag_define.glsl
index 2a91bde346..99ed5045e6 100644
--- a/packages/core/src/shaderlib/pbr/ibl_frag_define.glsl
+++ b/packages/core/src/shaderlib/pbr/ibl_frag_define.glsl
@@ -34,7 +34,7 @@ vec3 envBRDFApprox(vec3 specularColor,float roughness, float dotNV ) {
 
     vec2 AB = vec2( -1.04, 1.04 ) * a004 + r.zw;
 
-    // warning: (iPhone XS iOS 14.4) may return negative value.
+    // warning: add `max` because (iPhone XS iOS 14.4) may return negative value, refer to issue: https://github.com/galacean/engine/pull/1728
     return max(specularColor * AB.x + AB.y, 0.0);
 
 }