Why am I getting a build error about "Type 'String' is not a valid entity type" building this Silverlight web project?

Trying to build a Silverlight 4 application + website on a brand new machine and got the following error message for the website:

 Error Type 'String' is not a valid entity type.  Entity types cannot be a primitive 
 type or a simple type like string or Guid.

This doesn't make any sense at all since it 开发者_开发技巧builds on other machines and the Silverlight 4 SDK is installed on the new machine.

It's probably actually hiding this warning in Microsoft.Ria.Client.targets

The element 'PropertyGroup' in namespace 
'http://schemas.microsoft.com/developer/msbuild/2003' has invalid child element 
'RiaClientCodeGenDependsOn' in namespace 
'http://schemas.microsoft.com/developer/msbuild/2003'. 

Which most likely means there's a mismatch (or non-existent) version of WCF RIA Services for Silverlight 4.

Install the latest from http://www.silverlight.net/getstarted/riaservices and the problem should be solved.