How to do fog with HLSL

Just when I thought I understood HLSL there's always something to throw me back to zero. All I wanted to do is add fog to the water shader. So what I did was turning this:
Code:

---

 
struct VS_OUTPUT 
{
	float4 Pos: POSITION;
	float2 texCoord: TEXCOORD0;
	float3 eyeLinear: TEXCOORD1;
};
 

---

into this:
Code:

---

 
struct VS_OUTPUT 
{
	float4 Pos: POSITION;
	float2 texCoord: TEXCOORD0;
	float3 eyeLinear: TEXCOORD1;
	float fog: FOG;
};
 

---

If I'm already wrong here please tell me. Then (for testing) the vertex shader for calculating fog looks like this:
Code:

---

 Out.fog=1.0; 

---

Yes, I know it's stupid, but it's just for testing.
And my pixelshader just multiplies the output color with this value:
Code:

---

 reflection *= In.fog; 

---

But when I run it I get "error X4502: invalid reference to the input semantic 'POSITION0'". The line given is the declaration of the vertex shader output "float4 Pos: POSITION;".
Could anyone help me on this one? I really don't get it...

Anyone???