Simple Cg shader not compiling

I'm starting with cg shaders so I want to implement a simple one.

Heres my code:

 Shader "Custom/MyShader" {
     Properties {
         _MainTex ("Base (RGB)", 2D) = "white" {}
     }
     SubShader {
         Tags { "RenderType"="Opaque" }
         LOD 200
         
         CGPROGRAM
         #pragma surface surf Lambert
 
         sampler2D _MainTex;
 
         struct Input {
             float2 uv_MainTex : TEXCOORD0;
             float4 transp : COLOR;
         };
         
         void surf (Input aInput, inout SurfaceOutput o) {
             float4 c = tex2D(_MainTex, aInput.uv_MainTex) * aInput.transp;
             o.Albedo = c.rgb;
             o.Alpha = c.a;
         }
         ENDCG
     } 
     FallBack "Diffuse"
 }

If I delete " * aInput.transp" from the first line in surf, it compiles. I don't know why I can't multiply those values. Aren't they both float4?