ERROR in EPUB content.opf

[crankypants]

I'm getting an error in my epub2 content.opf. It's a simple EPUB I'm using to test validation. Here's the error:

Error while parsing file 'element "metadata" incomplete; missing required 
elements "ns:identifier", "ns:language" and "ns:title" (with 
xmlns:ns="http://purl.org/dc/elements/1.1/")'.

Here's the content.opf file. The error is line 13 position 12:

<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<package xmlns="http://www.idpf.org/2007/opf" unique-identifier="BookId" version="2.0">
<metadata xmlns:dc="http://purl.org/dc/elements/1.1" xmlns:opf="http://www.idpf.org/2007/opf">
<dc:title>My test title</dc:title>
<dc:language>en</dc:language>
<dc:identifier id="BookId" opf:scheme="ISBN">mytesttitle-20160601</dc:identifier>
<dc:creator opf:role="aut">John Smith</dc:creator>
<dc:creator opf:role="aut">Sally Jones</dc:creator>
<dc:subject>test subject</dc:subject>
<dc:subject>boats</dc:subject>
<dc:subject>travel</dc:subject>
<dc:date opf:event="modification">20160619</dc:date>
</metadata>
 
<manifest>
  <item id="ncx" href="toc.ncx" media-type="application/x-dtbncx+xml" />
  <item href="Text/ch1.xhtml" id="ch1.xhtml" media-type="application/xhtml+xml" />
  <item href="Text/ch2.xhtml" id="ch2.xhtml" media-type="application/xhtml+xml" />
  <item id="main-css" href="OEBPS/Style/note.css" media-type="text/css"/>
</manifest>
 
<spine toc="ncx">
  <itemref idref="ch1.xhtml" />
  <itemref idref="ch2.xhtml" />
</spine>
 
<guide>
  <reference href="Text/ch1.xhtml" title="Cover" type="cover" />
</guide>
</package>

I tried using Google to find a solution to the error but found no solution. Can someone help me? What are the formats of the data I'm missing in content.opf?

There's also another error in content.opf which is possibly related:

The unique-identifier 'BookId' was not found.

Thanks.

[Toxaris]

Your signature indicates that you use Sigil. Open the ePUB with Sigil and save it. That should fix this. It seems allright. What is reporting the error?

[dgatwood]

It might be dropping the identifier because the contents aren't valid, i.e. "mytesttitle-20160601" is not an ISBN.

[crankypants]

Originally Posted by dgatwood

It might be dropping the identifier because the contents aren't valid, i.e. "mytesttitle-20160601" is not an ISBN.

Nope. I have another EPUB made like that from another program that validates just fine.

I'm not using Sigil anymore so I took it out of my sig.

[jackie_w]

Another possibility to check ... the unique-identifier in the OPF is also supposed to match one of the <meta> items in the NCX file e.g.

<meta content="your_unique_identifier_here" name="dtb:uid"/>

[Doitsu]

@crankypants:

Your dc namespace attribute is missing the final slash:

<metadata xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:opf="http://www.idpf.org/2007/opf">

BTW, your modification date entry format is also invalid:

<dc:date opf:event="modification">20160619</dc:date>

You'll need to use:

<dc:date opf:event="modification">2016-06-19</dc:date>

[crankypants]

That was it Doitsu! Thank you!

That's one of those weird things that is hard to see.

[dgatwood]

Wow, that was subtle. Nice catch.