Silverlight Xaml error: "Length cannot be less than zero. Parameter name: length"

[This was originally posted at http://timstall.dotnetdevelopersjournal.com/silverlight_xaml_error_length_cannot_be_less_than_zero_pa.htm]

I was getting a strange Silverlight compile error in my Page.xaml the other day (while migrating stuff from 1.1 Alpha to 2.0 Beta):

Length cannot be less than zero.
Parameter name: length

At first it sounds like I was setting a wrong value - like trying to reference the "-1" position on a string. But, it actually was a constraint in what namespaces Xaml allows.

 

In 1.1 Alpha, this would be ok:

xmlns:Tank.Core="clr-namespace:Tank.Core;assembly=Tank.Core"

Which you could then reference like so:

However, that line kept throwing the error when I tried to compile.

 

It seems like if I remove the period ".", then it works again, like so:

xmlns:Tank1="clr-namespace:Tank.Core;assembly=Tank.Core"

Strange. I'm not fully sure why, maybe some parsing thing with periods "." in beta, but it was good to have a work-around.