Correctly passing a surf variable to custom Lighting<name> function

I'm finally getting specular working in my custom LightingPlanetSurface function but I can't seem to pass a particular variable into the function. I have the specular working fine, it's just when I try to include variables passed in from the surf function that the shine disappears.
In my surf function, I've got a _groundFlag variable that is set to 1 when a pixel is land and 0 when water.
Here's how my lighting function is set up:

1. struct SurfaceOutputCustom
   
2. {
   
3. fixed3 Albedo;
   
4. fixed3 Normal;
   
5. fixed3 Emission;
   
6. half Specular;
   
7. fixed Gloss;
   
8. fixed Alpha;
   
9. float4 flareColour;
   
10. float4 grainierColour;
    
11. float4 atmosphereShadowColour;
    
12. float distanceFactor;
    
13. float groundFlag;
    
14. };
15. half4 LightingPlanetSurface (SurfaceOutputCustom s, half3 lightDir, half3 viewDir, half atten) {
16. …. function details here
17. }

And my surf function is set up like this:

1. void surf (Input IN, inout SurfaceOutputCustom o) {
2. …_groundFlag variable decided here
3. // and I set the o.groundFlag variable like this...
4. o.groundFlag = _groundFlag;
5. }

The _groundFlag variable is set to a valid value; I use it for other calculations in the surf function.
So - why isn't it getting passed into the lighting function? When I include s.groundFlag in my specular calculations (even when I explicitly set the value to 1 or 0) the specular lighting disappears.
Any pointers, much appreciated, thanks.

 

I think I figured it out. One of my specular calculations was multiplying a half4 by a float and it looks like that was breaking it. So all sorted now.