Geometry Shader Only Outputs 3 Vertices

I'm having troubles rendering a billboard quad, that's generated from a single vertex in the geometry shader. For some reason I can only emit 3 vertices. Here's my shader:

[source lang="cpp"]struct QuadVertexInput {
float4 position_local : POSITION0;
float4 color : COLOR0;
};

struct QuadVertexOutput {
float4 position_projection : POSITION0;
float4 color : COLOR0;
};

struct QuadGeometryOut {
float4 position_screen : SV_Position;
float4 color : COLOR0;
};

struct QuadPixelOutput {
float4 color : SV_TARGET;
};

QuadVertexOutput QuadVertexMain(QuadVertexInput input) {
QuadVertexOutput output;
output.position_projection = mul(input.position_local, world_matrix);
output.color = input.color;
return output;
}

[maxvertexcount(4)]
void QuadGeometryMain(point QuadVertexOutput input[1], inout TriangleStream stream) {
QuadGeometryOut output = (QuadGeometryOut)0;

float3 y_axis = float3(0.0f, 1.0f, 0.0f);
float3 right = cross(y_axis, camera_direction);
float3 up = cross(camera_direction, right);

float3 center = input[0].position_projection;
float4 vertex0 = float4(center + up * 0.5f + right * 0.5f, 1.0f);
float4 vertex1 = float4(center - up * 0.5f + right * 0.5f, 1.0f);
float4 vertex2 = float4(center - up * 0.5f - right * 0.5f, 1.0f);
float4 vertex3 = float4(center + up * 0.5f - right * 0.5f, 1.0f);

vertex0 = mul(mul(vertex0, view_matrix), projection_matrix);
vertex1 = mul(mul(vertex1, view_matrix), projection_matrix);
vertex2 = mul(mul(vertex2, view_matrix), projection_matrix);
vertex3 = mul(mul(vertex3, view_matrix), projection_matrix);

output.position_screen = vertex0;
output.color = float4(1.0f, 0.0f, 0.0f, 1.0f);
stream.Append(output);

output.position_screen = vertex1;
output.color = float4(0.0f, 1.0f, 0.0f, 1.0f);
stream.Append(output);

output.position_screen = vertex2;
output.color = float4(0.0f, 0.0f, 1.0f, 1.0f);
stream.Append(output);

output.position_screen = vertex3;
output.color = float4(1.0f, 1.0f, 0.0f, 1.0f);
stream.Append(output);
}

QuadPixelOutput QuadPixelMain(QuadGeometryOut input) {
QuadPixelOutput output;
output.color = input.color;
return output;
}

technique10 Quad {
pass P0 {
SetVertexShader(CompileShader(vs_4_0, QuadVertexMain()));
SetGeometryShader(CompileShader(gs_4_0, QuadGeometryMain()));
SetPixelShader(CompileShader(ps_4_0, QuadPixelMain()));
}
}[/source]

Can anyone help me out with what I'm doing wrong?

Here's an image of what it looks like:

http://imageshack.us/photo/my-images/836/picnj.png/