Can't validate simple xml file and schema!

Using the xml-schema validator at:
http://www.w3.org/2000/09/webdata/xsv

Highly misleading error message (I am tearing my
hair in desperation!);
Error: Expected whitespace after public id
 in unnamed entity at line 1 char 59 of http://www.owalter.co.uk/stest1b
.xml
this message remained unchanged even when I modified both files (add,
remove comments in <!-- --> and added an undeclared tag in the xml file.

THE XML FILE
<?xml version="1.0"?>
<!-- first test of validation with a schema.
This .xml file does not contain the name of the schema -->
<a_tag>Tag contents is this text.
<!-- comment this out for the time being
 <contained_tag this_att="a_value"/> -->
</a_tag>

THE SCHEMA
<?xml version="1.0"?>
<!-- schema for first simple test -->
<xs:schema xmlns:xsd="http://www.w3.org/2000/10/XMLSchema">
  <xs:element name="a_tag" type="xs:string"/>
</xs:schema>

From

oliver@owalter.co.uk
-- 
Oliver Walter