XSLT for-each-group from variable does not work - grouping

Question

for-each-group from XSLT 2.0 works as expected from a file but not from a variable.
Have this file:
~$ cat test.xml
<?xml version="1.0" encoding="UTF-8"?>
<root>
<delimiter/>
<c>A</c><c>B</c>
<delimiter/>
<c>C</c>
</root>
Using stylesheet for grouping this file:
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="2.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" version="1.0"
encoding="UTF-8" indent="yes" omit-xml-declaration="no" />
<xsl:template match="*">
<!-- variable not used for file test -->
<xsl:variable name="fields">
<root>
<delimiter/>
<c>A</c><c>B</c>
<delimiter/>
<c>C</c>
</root>
</xsl:variable>
<xsl:for-each-group select="*" group-starting-with="delimiter">
<field>
<xsl:for-each select="current-group()">
<xsl:value-of select="self::c"/>
</xsl:for-each>
</field>
</xsl:for-each-group>
</xsl:template>
</xsl:stylesheet>
I get the result I want:
<?xml version="1.0" encoding="UTF-8"?>
<field>AB</field>
<field>C</field>
Trying to group the variable name="fields" with:
<xsl:for-each-group select="$fields/*" group-starting-with="delimiter">
I get the result:
<?xml version="1.0" encoding="UTF-8"?>
<field/>
Why does for-each-group works on a file but not from a variable?

Answer 1

The variable fields is a document-node(), you can define the type of the variable to be element()
<xsl:variable name="fields" as="element()">
<root>
<delimiter/>
<c>A</c><c>B</c>
<delimiter/>
<c>C</c>
</root>
</xsl:variable>