Unknown Build Error 'key cannot be null'

We are working on this project in a team and I just copied the line for using resources... I just forgot to copy the xmlns attribute as well. What I find strange is that the error isn't really descriptive and doesn't give any real hints as to what the problem is.

Moral of the story: if copying lines of code make sure that all references to namespaces are also copied.

I was getting the same exact error and it turned out to be something different for me.

I was binding to properties using a fully qualified syntax because I didn't have a target object in my binding. However I was doing it with shorthand and that's apparently not allowed with fully qualified syntax. I had something like this:

{Binding (cmn:ElementData.ID)}

I changed it to:

{Binding Path=(cmn:ElementData.ID)}

And boom, cryptic error gone and everything worked as expected. Apparently VERY important to not use shorthand with this...

Thanks to Andrew Stakhov at this link whose comment tipped me off to this.