'discard' keyword cg error

Hey all,

I'm trying to implement alpha testing in one of my shaders but am getting a strange error. When using the 'discard' keyword in the fragment shader to alpha test I get this error on compilation -

GLSL vertex shader: ERROR: 0:325: 'discard' : supported in fragment shaders only at line 24

I am definitely using the 'discard' keyword in the fragment shader and not in the vertex shader so this makes no sense to me.

Full shader source follows, thank you for any help in advance.

 SubShader {
         Tags {"Queue"="AlphaTest" "IgnoreProjector"="True" "RenderType"="TransparentCutout"}
         LOD 100
         
         Pass {
             CGPROGRAM
             #pragma vertex vert
             #pragma fragment frag 
             
             #include "UnityCG.cginc"
             
             //Global
             sampler2D _backgroundLighting;
             float4x4 _ProjConst;
             
             float4 _ambientColor;
             float4 _shadowColor;
             
             //Properties
             sampler2D _MainTex;
             
             struct v2f {
                 float4 pos: SV_POSITION;
                 float2 uv : TEXCOORD0;
                 float4 uv2 : TEXCOORD1;
             };
             
             float4 _MainTex_ST;
             
             v2f vert (appdata_base v) {
                 v2f o;
                 o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
                 o.uv = TRANSFORM_TEX(v.texcoord, _MainTex);
                 
                 //Screen space uv
                 o.uv2 = mul(_ProjConst, o.pos);
                 
                 return o;
             }
             
             half4 frag (v2f i) : COLOR {
                 half4 c = tex2D(_MainTex, i.uv);
                 half4 l = tex2D(_backgroundLighting, i.uv2.xy);
                 
                 half4 shad = c * _shadowColor;
                 
                 if(c.w < 0.5) {
                     discard;
                 }
                 
                 return half4((lerp(shad, c, l.w) + _ambientColor).xyz, c.w);
             }
             ENDCG
         }
     }