Oracle UpdateXML not accepting a valid xPath

I have an XML in XMLTYPE column, that looks like the following:

<n:ROOT xmlns:app="http://company/acms/content/DocType" xmlns:n="http://company/acms/content/DocType">
   <Content>
      <Definition>
         <Code>DocType</Code>
         <Version>1</Version>
      </Definition>
      <DocType>
         <Info>
            <DocumentType>DocType</DocumentType>
            <DocumentClass>Other</DocumentClass>
            <CustomerID>12323423</CustomerID>
            <FinancialAccountID>12312312</FinancialAccountID>
            <MSISDN>34534534</MSISDN>
            <CustomerType>Consumer</CustomerType>
            <CustomerSubType>Consumer-Platinum</CustomerSubType>
            <FirstName>Name</FirstName>
            <ServiceType>ServiceType</ServiceType>
            <DocumentSource>BadValueHere</DocumentSource>
            <BoxReferenceNumber>BoxXXX</BoxReferenceNumber>
            <Industry>OTHERS</Industry>
            <CreationDate>2015-01-01</CreationDate>
            <DocumentID>6666</DocumentID>
            <CreatedBy>UBIX</CreatedBy>
            <DocumentOrigin>HD</DocumentOrigin>
            <Notes>Document location on File System: /path/to/doc.TIF</Notes>
            <Resource parseType="none">
               <URL>/path/filename.pdf</URL>
               <FileName>filename.pdf</FileName>
               <MimeType>image/tiff</MimeType>
            </Resource>
            <FileType>tiff</FileType>
         </Info>
         <Permissions>
            <GroupPermissions>
               <GroupName>99</GroupName>
               <Permissions>C</Permissions>
            </GroupPermissions>
            <GroupPermissions>
               <GroupName>30</GroupName>
               <Permissions>W</Permissions>
            </GroupPermissions>
            <GroupPermissions>
               <GroupName>66</GroupName>
               <Permissions>R</Permissions>
            </GroupPermissions>
         </Permissions>
      </DocType>
   </Content>
</n:ROOT>

Now I need to update the DocumentSource element, to a new value.

I tried the following query:

SET CONTENT_DATA =  UPDATEXML(CONTENT_DATA,
   '/n:ROOT/Content/DocType/Info/DocumentSource/text()', 'Test')
where CONTENT_ID=6666;

However, when I try to run it, I get Oracle error: SQL Error: ORA-31013: Invalid XPATH expression 31013. 00000 - "Invalid XPATH expression" *Cause: XPATH expression passed to the function is invalid. *Action: Check the xpath expression for possible syntax errors.

Documentation for UpdateXML is on: https://docs.oracle.com/cd/B19306_01/server.102/b14200/functions205.htm it talks about 5th element to specify the namespace, but I have had no luck getting it to work :( There are absolutely zero examples on how to use it in my scenario on Google...

Can any expert please help?

Just try the xpath : //Content/DocType/Info/DocumentSource/text()