For what it's worth, I've attached a dummy epub which shows the error when linear="no" is used in the spine entry for the nav.xhtml file. Removing the linear="no" or changing to linear="yes" does not display the error. I also played with changing the ID and that did not help. Removing the line from the spine entries also worked.
I did find that adding the following to the nav.xhtml file cleared the error:
Code:
<nav epub:type="landmarks"><h2>Guide</h2>
<ol>
<li><a epub:type="toc" href="nav.xhtml"> Table of Contents </a></li>
</ol></nav>
Probably since it supplies the hyperlink for the nav.xhtml file.