Problem running Unit Tests with VB.NET

Hello,

we just got installed R# 5.1 (Build 5.1.1727.12) for VB.NET. When running our Unit Tests with R# we got always the following error:

Unit Test Runner failed to load test assembly:
JetBrains.ReSharper.TaskrunnerFramework.TaskException: Method not found: Void
Microsoft.CSharp.CSharpCodeProvider..ctor(System.Collections.Generic.IDictionary`2<System.String,System.String>).

I'm a bit concerned about the Word CSharp in the error message as we only got the VB version of R#.
What seems to be the problem?

Regards,
Aline