XSL - Test a boolean value

I've got a problem with a XSLT code in SharePoint 2010. I'm not very expierienced with XSLT, but I think, I am able to read the code. From the SharePoint query I got this XML code:

<Result>
    <id>2</id>
    <workid>160</workid>
    <rank>67119018</rank>
    <title>Angebot.pdf</title>
    <author_multival>User, Name</author_multival>
    <author>User, Name</author>
    <size>440631</size>
    <url>http://ift.tt/1vyKqya;
    <urlEncoded>http%3A%2F%2Fportal%2Eurl%2Ede%2Fsites%2FIT%2FProjects%2FAngebot%2Epdf</urlEncoded>
    <description></description>
    <write>21.03.2014</write>
    <sitename>http://ift.tt/1EQhRfF;
    <collapsingstatus>0</collapsingstatus>
    <hithighlightedsummary>User, Name <ddd /> <c0>SharePoint</c0>:Z-Drive <ddd /> Costs</hithighlightedsummary>
    <hithighlightedproperties>
            <HHTitle>Angebot.pdf</HHTitle>
            <HHUrl>http://ift.tt/1vyKnT4;
    </hithighlightedproperties>
    <contentclass>STS_ListItem_DocumentLibrary</contentclass>
    <isdocument>True</isdocument>
    <picturethumbnailurl></picturethumbnailurl>
    <serverredirectedurl></serverredirectedurl>
    <sharepoint-invalid>False</sharepoint-invalid>
    <fileextension_multival>PDF</fileextension_multival>
    <fileextension>PDF</fileextension>
    <ows_metadatafacetinfo>Projectx0020Name|Project Name|o3PIYhfa0kOegAKxi60qtQ==|S7ZoMTb2ikCAhuSFlC+sGA==|#43318cf9-39f5-461c-a4d0-bd2653612092|SharePoint:Z-Drive;#Projectx0020Documentx0020Category0|Project Document Category|o3PIYhfa0kOegAKxi60qtQ==|zRewJpp710mnJ0bWknAb2Q==|#d8f09524-a0dc-4324-97c3-1d566e4af75f|Costs;#</ows_metadatafacetinfo>
    <popularsocialtags />
    <imageurl imageurldescription="">/_layouts/images/pdf16.gif</imageurl>

Now I need to check the value of

<sharepoint-invalid>False</sharepoint-invalid>

in XSLT. I tried the following code, but it won't work. I don't know how to check the boolean values correctly, because in the search is always "Valid" displayed.

Here is my XSLT code:

<xsl:choose>
    <xsl:when test="$SharePoint-Invalid = 'True'">
            <span style="color:#EF1317;font-weight:bold;">Invalid</span>
    </xsl:when>
    <xsl:otherwise>
            <span style="color:#199039;font-weight:bold;">Valid</span>
    </xsl:otherwise> </xsl:choose>

Can anyone help? Thanks in advance.