Simple Shader Syntax ... or so I thought

The following shader code throws an error.
void SurfaceProgram(Input IN, inout SurfaceOutput OUT)
{
//OUT.Alpha = 1;//This works fine
OUT.Alpha = IN.uv_textureName.x;
}

The error is: "Program 'frag_SurfaceProgram', assignment of incompatible types"

I believe that it is because unity treats uvs differently from normal floats (or a half in the case of SurfaceOutput.Alpha).

Is there any way of telling unity to treat them as normal variables? Some sort of cast or #pragma maybe?