Unity cutoff shader has texture's all channels always 0 - unity3d

Question

I am trying to write a cut off shader which utilizes a _MainTex in a ShadowCaster pass where I have the pass:
Pass {
Name "ShadowCaster"
Tags {
"LightMode"="ShadowCaster"
}
Offset 1, 1
Cull Off
CGPROGRAM
#pragma vertex vert
#pragma fragment frag
// ...
#pragma multi_compile_shadowcaster
// ...
// structs and variables
// ...
}
The frag shader is like:
float4 frag(VertexOutput i, float facing : VFACE) : COLOR {
// no shadow on the pixel if alpha less than a cutoff
float4 texcol = tex2D( _MainTex, i.uv1.xy );
// but it's always = 0, so everything is cutoff
clip(texcol.a * _Color.a - _Cutoff);
// it returns 0, which doesn't matter
SHADOW_CASTER_FRAGMENT(i)
}
The texcol always has 0 for alpha channel as I can test with clip(texcol.a - 0.0) and modify the number. And the value I used in tex2D is straightly a copy from previous pass:
VertexOutput vert (VertexInput v) {
VertexOutput o = (VertexOutput)0;
o.uv1 = v.texcoord1;
// ...
TRANSFER_SHADOW_CASTER(o)
return o;
}
And the structs are:
struct VertexInput {
float4 vertex : POSITION;
float2 texcoord1 : TEXCOORD1;
float2 texcoord2 : TEXCOORD2;
}
struct VertexOutput {
V2F_SHADOW_CASTER;
float2 uv1 : TEXCOORD1;
float2 uv2 : TEXCOORD2;
float4 posWorld : TEXCOORD3;
}
What is wrong with the texture transformation I used in tex2D? Or is it something else? I am sure the texture itself does provide correct alpha value.
Update. The full shader pass is on gist, there's the relevant part: https://gist.github.com/KHN190/0fd1d2bab899fbe2fcc809c6755b3d74