Why do I get “The property 'Pages' does not have an accessible setter” for a simple <FixedDocument> in Expression Blend 4?

Question

I have a XAML file which I have pared down to find the cause of this issue to the following minimal case:

<FixedDocument
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" >

    <PageContent>

    </PageContent>
</FixedDocument>

It opens fine in a KAXAML, but when I open the same file in Expression Blend 4, I get the following 2 errors at the <PageContent> tag:

• The property 'Pages' does not have an accessible setter
• The specified value cannot be assigned. The following type was expected: 'PageContentCollection'.

There is no CodeBehind. The only interesting thing is that this file exists in a Winforms project, and I have applied the following hack: http://social.expression.microsoft.com/Forums/en-US/blend/thread/c726522f-c721-42a7-8bf1-c73360edaa60 - but the problem also happens with the file in a straight WPF project.

Does anyone know why, and have a fix for this?