Compiling a shader with fxc results in invalid ps_5_0 output semantic 'COLOR0'

I'm attempting to compile a shader at the command prompt. What am I doing wrong that would make it generate this error?

fxc /Od /Zi /T ps_5_0 /E "ps_main" /Fo "basic.pso" "basic.ps"

Here is the pixel shader

struct VS_OUTPUT
{
    float4 Color    : COLOR0;
};


float4 ps_main(VS_OUTPUT input) : COLOR0
{
    return input.Color;
}

The return is:

<path here>\basic.ps(8,33-40): error X4502: invalid ps_5_0 output semantic 'COLOR0'

compilation failed; no code produced