Custom Build Activities and TF215097 Error

TFS 2010 Build Automation can be customized by creating custom WF activities.
The net is already full with how-to articles like this one:
http://blogs.msdn.com/b/jimlamb/archive/2009/11/18/how-to-create-a-custom-workflow-activity-for-tfs-build-2010.aspx

When I tried to queue the build, I got this annoying TF215097 error:
 

TF215097: An error occurred while initializing a build for build definition "Cannot create unknown type '{clr-namespace:BuildTasks;assembly=BuildTasks}'"

To solve this, you need to open the WF XAML build in notepad and check the xnmns attribute ofthe custom activity.
The Visual Studio IDE only uses:
clr-namespace:BuildTasks
Instead of:
clr-namespace:BuildTasks;assembly=BuildTasks

So, just add ;assembly=BuildTasks and it will work for you too.