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Error explainers · XML

ParseError: not well-formed (invalid token)

Expat, the parser behind Python’s ElementTree and Firefox, uses one generic message for many problems: it found a character that cannot appear where it is. The column points at that character. The common culprits are an attribute value without quotes, a raw < or & in text, a tag name that starts with a digit, and invisible control characters. PasteKit names the specific rule that was broken.

Seen as:

  • xml.etree.ElementTree.ParseError: not well-formed (invalid token): line 2, column 15
  • xml.parsers.expat.ExpatError: not well-formed (invalid token): line 2, column 15
  • XML Parsing Error: not well-formed
  • parser error : AttValue: " or ' expected
  • parser error : StartTag: invalid element name

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Common causes

1. An attribute value without quotes

HTML allows port=8080; XML requires every attribute value in single or double quotes.

Before
<server port=8080 host="localhost"/>
After
<server port="8080" host="localhost"/>

2. A raw < in text content

< always starts a tag in XML. Write it as &lt; in text and attribute values (and > as &gt; for symmetry), or wrap code in a CDATA section.

Before
<limit>maxConnections < 100</limit>
After
<limit>maxConnections &lt; 100</limit>

3. An invalid element or attribute name

Names must start with a letter, underscore or colon, and cannot contain spaces. Keys converted from JSON or spreadsheet headers often break this rule.

Before
<user><2fa>true</2fa><first name>Ada</first name></user>
After
<user><twoFactor>true</twoFactor><firstName>Ada</firstName></user>

4. Control characters in the text

XML 1.0 forbids most characters below U+0020, such as the vertical tab and form feed Word and old databases produce. Even &#11; is not allowed. Remove them before generating the XML.

Before
<note>Line oneLine two</note>
After
<note>Line one Line two</note>

Frequently asked questions

Why is the message so vague?

Expat reports every illegal character with the same error code, XML_ERROR_INVALID_TOKEN. The line and column are accurate, so look exactly there; PasteKit explains which rule the character breaks.

Can a wrong encoding cause this error?

Yes. A file saved as Windows-1252 but declared or read as UTF-8 contains byte sequences that are not valid UTF-8, and expat reports them as an invalid token. Re-save the file as UTF-8 or declare the real encoding.

What about a raw & in text?

Expat reports a bare & as not well-formed (invalid token) too. See unescaped ampersands in XML for that case.

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