Shader Error: multiplying worldNormal * texture.rgb

I was trying to use worldNormals and NormalMap into a surface shader, but I came into an error where I can't use in the same equation the worldNormal and the texture's rgb values (either directly or indirectly).

Here's the simplest shader I could make so you can see the error by yourselves:

 Shader "MyShaders/TestShader" {
 
     Properties {
       _MainTex ("Diffuse Map", 2D) = "white" {}
       _NormalMap ("Normal Map", 2D) = "bump" {}
     }
     
     SubShader {
       Tags { "RenderType" = "Opaque" }
       CGPROGRAM
       #pragma surface surf Custom
 
       half4 LightingCustom( SurfaceOutput s, half3 lightDir, half3 viewDir, half atten)
       {
           return half4(0,0,0,0);
       }
       
       struct Input
       {
           float2 uv_MainTex;
           float2 uv_NormalMap;
           float3 worldNormal;
           INTERNAL_DATA
       };
       
       sampler2D _MainTex;
       sampler2D _NormalMap;
       void surf(Input IN, inout SurfaceOutput o)
       {
              half3 worldNormal = WorldNormalVector( IN, float3(0,0,1) );
              half3 dN = dot( worldNormal, half3(1,0,0) );
              
           o.Albedo = tex2D(_MainTex, IN.uv_MainTex).rgb * dN;  // ERROR HERE: sends error when multiplied * dN, otherwise it doesn't
           o.Normal = UnpackNormal( tex2D(_NormalMap, IN.uv_NormalMap) );
       }
       ENDCG
     }
     Fallback "Diffuse"
 }

Is there a reason why if I multiply ".rgb dN" sends a compile error, and if I just use ".rgb" alone or multiply it by any constant (like ".rgb 0.5") it works ok?..

is this a bug of the compiler? or am I doing something wrong?

Thanks!