_CameraDepthTexture in vertex: tesselation dx11 2017.10 (forward shader)

Hi

I tried to get the _CameraDepthTexture in vertex:disp but that cause an error:

Code (CSharp):

1. SubShader{
2.        Tags{ "Queue" = "Transparent" }
3.        LOD 300
4.        Zwrite On
5.        GrabPass{ "_GrabTexture" }
6.        CGPROGRAM
7. #pragma surface surf StandardSpecularWater vertex:disp tessellate:tessDistance nolightmap keepalpha nofog
8.  
9. #pragma target 4.6
10. #include "Tessellation.cginc"
11.  
12. struct Input {
13.        float3 worldPos;
14.        float4 screenPos;
15.        float3 worldNormal;
16.        INTERNAL_DATA
17.    };
18.  
19. float4 tessDistance(appdata v0, appdata v1, appdata v2)
20.     {
21.         return UnityDistanceBasedTess(...);
22.     }
23.  
24. void disp(inout appdata v)
25.     {
26.              float depth = LinearEyeDepth(tex2Dlod(_CameraDepthTexture, float4(screenPos, 0.0, 0.0)).r);
27.    // undeclared identifier '_CameraDepthTexture' at line 149 (on d3d11)
28.              half edgeBlendFactor = saturate(_InvFadeParameter * (depth - screenPos.w));
29.     }

undeclared identifier '_CameraDepthTexture' at line 149 (on d3d11)

Id like to smooth out the vertex height based on the depth info.

How can I get this lines of code to work?
Thank you..