SPListItem provided is not compatible with a Publishing Page

I have been hitting my head against a wall lately with a custom master page and some custom sites. There appears to be an incompatibility between the 2. Since they come from separate sources, finding out where to point the finger is a little tricky. My custom master page makes use of Publishing, Feature Stapling, among other things.

The behaviour is that when I edit a community page and then click publish, it would throw an exception

System.ArgumentException: Invalid SPListItem. The SPListItem provided is not compatible with a Publishing Page.   
 at Microsoft.SharePoint.Publishing.PublishingPage.GetPublishingPage(SPListItem sourceListItem)    
 at Microsoft.SharePoint.Publishing.Internal.WebControls.PublishingPagePublishHandler.RaisePostBackEvent(String eventArgument)    
 at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument)    
 at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

I have yet to figure out what is going on here. However here are some other observations

$web = Get-SPWeb <SPWeb URL>
[Microsoft.SharePoint.Publishing.PublishingWeb]::IsPublishingWeb($web)

This returns false despite the Publishing tab being there and the SharePoint Server Publishing site feature is activated.


So, ignoring the False, I continue with this:
$publishingweb = [Microsoft.SharePoint.Publishing.PublishingWeb]::GetPublishingWeb($web)
$publishingpages = $publishingweb.GetPublishingPages()

This throws an exception:

Exception calling "GetPublishingPages" with "0" argument(s): "The site is not valid. The 'Pages' document library is missing."
At line:1 char:53
+ $publishingpages = $publishingweb.GetPublishingPages <<<< ()
    + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException
    + FullyQualifiedErrorId : DotNetMethodException

... but the Pages document library does indeed exist and I do have administrative rights on it.


I also try the following:

$pages = $web.Lists | Where-Object {$_.Title -eq "Pages"}
$page = $pages.Items | Where-Object {$_.Name -eq "default.aspx"}
[Microsoft.SharePoint.Publishing.PublishingPage]::IsPublishingPage($page)

... and get False which I would expect given the results from above. What doesn’t make sense is that this default.aspx page does have a Publish tab on it.

I then decided to try the IsPublishingWeb again, but without the custom master page. This is still giving me False. So, I guess I will start pointing the finger towards my custom site ...