Why is this simple STEP file invalid?

Question

So, I have what I believe is a simple and valid AP203 step file. However it fails to load in Fusion360 and I cannot seem to get the NIST validator to give me any useful info.

Does anyone understand what is invalid about this file:

============= snip =====================

# AP203 STEP file
ISO-10303-21;

HEADER;
  FILE_DESCRIPTION("Simple line STEP file");
ENDSEC;

DATA;

  # Line definition
  ENTITY line
    SUBTYPE OF (straight_curve);
    point1 = (0, 0, 0);
    point2 = (1, 1, 0);
  END_ENTITY;

ENDSEC;

ENDISO;

I've tried to load it in several tools but simply get innocuous messages...

Fusion360 sez simply:

Autodesk Translation Services failed to translate the step file, please make sure the step file isn't corrupt!

SolidWorks sez:

the file is invalid, not found, locked or invalid

The NIST validator is a little better but still makes no sense:

can't read "fsline": no such variable while executing "string first "(" $fsline" (procedure "getSchemaFromFile" line 88)

The Autodesk Viewer claims that the file has no geometry. I have also tried this file format with no success:

ISO-10303-21;
HEADER(#2);
  FILE_DESCRIPTION(('ACAP', '2024-03-02', 'Example Generator', 'v1.0'));
ENDSEC;
DATA (#3);
  # Define a line
  ENTITY(edge_curve);
    name = 'MyLine';
    description = 'A line from (0,0) to (1,1)';
    line = (
      (cartesian_point(coordinates = (0.0, 0.0, 0.0))),
      (cartesian_point(coordinates = (1.0, 1.0, 0.0)))
    );
  END_ENTITY;
ENDSEC;
END-ISO-10303-21;