Cannot select a node here: the context item is unde

Again thank you all for your great responses. I just about have this 
project completed.

I am using Saxon 8.3

the piece of the stylesheet giving the error message is >>>marked<<<<<
        <xsl:key name="word-group-by-word" match="word-group" use="word"/>
        <xsl:function name="str:find_root_word" as="xs:string">
                <xsl:param name="word" as="xs:string"/>
                <xsl:variable name="length-of-word" select="
string-length($word)"  />
                <xsl:choose>
                        <!-- this should remove the 'ed' -->
>>>>>   <xsl:when test="key('word-group-by-word',substring($word,1, 
$length-of-word - 2))"> <<<<<<<<<<<<<<<<<<
                                <xsl:sequence select="substring($word,1, 
$length-of-word - 2)"/>
                        </xsl:when>

So what is the error? Cannot select a node here: the context item is 
undefined
 
What I am trying to do.
I have words in word-group that I am trying to analyze. If the 
master-count is non-zero then I am happy that we have the word found in a 
Bible and in the master list e.g., falter.

If the master-count is null then it should be flagged as a possible error. 
But since  the last three words are only variations of the first word 
listed below I would like to exclude them as problems. 
        <word>falter</word>
        <word>faltered</word>
        <word>faltering</word>
        <word>falters</word>


piece of xml file:
<word-group>
        <word>falter</word>
        <master-count>1</master-count>
        <master-number>9.4.3.1.5; </master-number>
        <master-name>Uncertain; </master-name>
        <niv-count>2</niv-count>
        <nasb-count>1</nasb-count>
        <nlt-count>1</nlt-count>
        <tev-count/>
        <niv-references>PRO 24:10 ISA 42:4</niv-references>
        <nasb-references>LEV 25:35</nasb-references>
        <nlt-references>MAT 21:26</nlt-references>
        <tev-references/>
</word-group>
<word-group>
        <word>faltered</word>
        <master-count/>
        <master-number/>
        <master-name/>
        <niv-count>1</niv-count>
        <nasb-count/>
        <nlt-count/>
        <tev-count/>
        <niv-references>PSA 105:37</niv-references>
        <nasb-references/>
        <nlt-references/>
        <tev-references/>
</word-group>
<word-group>
        <word>faltering</word>
        <master-count/>
        <master-number/>
        <master-name/>
        <niv-count>3</niv-count>
        <nasb-count/>
        <nlt-count/>
        <tev-count/>
        <niv-references>EXO 6:12,30 JOB 4:4</niv-references>
        <nasb-references/>
        <nlt-references/>
        <tev-references/>
</word-group>
<word-group>
        <word>falters</word>
        <master-count/>
        <master-number/>
        <master-name/>
        <niv-count>1</niv-count>
        <nasb-count/>
        <nlt-count/>
        <tev-count/>
        <niv-references>ISA 21:4</niv-references>
        <nasb-references/>
        <nlt-references/>
        <tev-references/>
</word-group>


Jim Albright
704 843-0582
Wycliffe Bible Translators