Hide HTML section in WordProcessingDocument object using OpenXML's Vanish Class

I am attempting to append an HTML page into a DocM file, and have the entire content hidden. As I understand, I need to use the Vanish class (https://docs.microsoft.com/en-us/dotnet/api/documentformat.openxml.wordprocessing.vanish?view=openxml-2.8.1) to achieve this.

I streamed the HTML content into an AltChunk using the code provided here: https://stackoverflow.com/a/18152334/1243462 . I then try to append the altchunk to an instance of the Vanish class, and then add that Vanish object to the MainBodyPart, like so:

Vanish hideThisHTMLContent = new Vanish();
string altChunkId = "altChunk1";

// Create alternative format import part.
AlternativeFormatImportPart chunk = copy.MainDocumentPart.AddAlternativeFormatImportPart
  (AlternativeFormatImportPartType.Html, altChunkId);

using MemoryStream htmlContentStream = new MemoryStream(new UTF8Encoding(true)
  .GetPreamble().Concat(Encoding.UTF8.GetBytes(attachmentContent)).ToArray());

// Feed HTML data into the chunk.
chunk.FeedData(htmlContentStream);
AltChunk altChunk = new AltChunk
{
    Id = altChunkId
};

//Hide the HTML alt chunk by appending it as a child to the hidden element
hiddenPart.AppendChild(altChunk);

//Append hidden part to MainDocumentPart.Body
copy.MainDocumentPart.Document.Body.Append(hiddenPart);

My understanding is that this append is needed so that the HTML content "inherits" the hidden property of the object. This code compiles, but I get a runtime error, as shown below:

System.InvalidOperationException
HResult=0x80131509
Message=Non-composite elements do not have child elements.
Source=DocumentFormat.OpenXml
StackTrace:
 at DocumentFormat.OpenXml.OpenXmlElement.AppendChild[T](T newChild)
 at ApproachTwoAccessWordElements.Program.doUsingHiddenClass() in 
  C:\Users\sraje\source\repos\ApproachTwoAccessWordElements\Program.cs:line 55
 at ApproachTwoAccessWordElements.Program.Main(String[] args) in 
  C:\Users\sraje\source\repos\ApproachTwoAccessWordElements\Program.cs:line 16

Can someone please help with this? I am also unable to find code samples for how to properly hide sections using this property