Build AsmMeta failing because it’s unable to get a reference to an object

Today I was working with some Windows Mobile project and that required me to add some XMTA files and everything was working just fine until I add a new XMTA file.

When I hit the build button, the build failed with the following message:

There was an error finalizing type. 
Object reference not set to an instance of an object.

I've spent the last hour searching over the Net to find a solution. But everything that I found about the failure of generating assembly meta-data was in reference to some method that doesn't implemented an abstract method, or similar.

It was only in a comment from Alex Van Den Bosch, in Morten Nielsen's blog that I (out of desperation) found the solution what appeared to be the:

"change the build action of the XMTA to Content"

After I changed this the error didn't show up, but the assembly meta-data does not get created.

Well... back to the drawing board...