cross product in cg shader - error "can't find __getVectorIndex"

Anyone know what this is about? This is my first real Unity shader (it's a port of a cg shader from a different project I'm moving to Unity). Here's the shader:

 Shader "Starboretum/Flora" {
     Properties {
            _MainTex ("Texture", 2D) = ""
     }
     SubShader {
         
         Pass {
         CGPROGRAM
 
 #pragma vertex vert
 #pragma fragment frag
 #include "UnityCG.cginc"
 
 sampler2D _MainTex;
 float4 _MainTex_ST;
 
 struct appdata {
     float4 vertex : POSITION;
     float3 normal : NORMAL;
     float4 tangent : TANGENT;
     float4 texcoord : TEXCOORD0;
     fixed4 color : COLOR;
 };
 
 struct v2f {
     float4     pos : SV_POSITION;
     float2  uv : TEXCOORD0;
     fixed4 color : COLOR;
 };
 
 const float4 uCoords = {0,1,1,0};
 const float4 vCoords = {0.5f,0.5f,0.5f,0.5f};
 const int4 vCEnd = {0,0,1,1};
 const int4 vCStart = {1,1,0,0};
 const int4 wStart = {0,0,-1,1};
 const int4 wEnd = {1,-1,0,0};
 
 v2f vert (appdata v)
 {
     int corner = (int)(v.tangent.w);
     
 
     float3 prev = v.tangent.xyz;
     float3 start = v.vertex.xyz;
     float3 end = v.normal.xyz;
 
 
     float3 dif1 = (start - prev);
     float3 dif2 = end - start;
     
 
     // get the direction of the line, using the previous or next segments as a guide depending on which end we're at 
     float3 dir = dif1 * vCStart[corner] + dif2 * vCEnd[corner];
     
 
     float3 d1 = _WorldSpaceCameraPos - start;
     float3 d2 = _WorldSpaceCameraPos - end;
 
     // get a vector from the segment to the bamera
     float3 camDif = d1 * vCStart[corner] + d2 * vCEnd[corner];
 
     // obtain the correct width of the segment
     float widthStart = v.texcoord.x;
     float widthEnd = v.texcoord.y;
     float width = widthStart * wStart[corner] + widthEnd * wEnd[corner];
     
 
     // get a vector perpendicular to the direction to the camera
     float3 left = normalize(cross(dir, camDif));
     //  expand the line segment by the width, and add on the length if it's one of the end verts
     v.vertex.xyz += left * width + dif2 * vCEnd[corner];
 
     // insert correct UVs
     v.texcoord.x = uCoords[corner];
     v.texcoord.y = vCoords[corner];
         
     v2f o;
     o.pos = mul (UNITY_MATRIX_MVP, v.vertex);
     o.uv = TRANSFORM_TEX (v.texcoord, _MainTex);
     o.color = v.color;
     return o;
 }
 
 half4 frag (v2f i) : COLOR
 {
     half4 texcol = tex2D (_MainTex, i.uv);
     return texcol * i.color;
 }
 
         ENDCG
         }
     } 
     FallBack "VertexLit"
 }

The line

 float3 left = normalize(cross(dir, camDif));

Gives the error

 Shader error in 'Starboretum/Flora': Program 'vert', can't find __getVectorIndex function in stdlib at line 1313

When Unity tries to compile the shader.

I know it's that line because I commented stuff out and put it back in until the error reappeared and it was that line. I also tried taking out the normalize and it still happened.

Any ideas?

Thanks.