After checking out the error for past users, is this happening when you are debugging Silverlight customized to code test steps? And if you are, can you post the code that was generated and any modifications you made to the code?
If you are debugging code, please try removing any breakpoints and seeing if the test runs correctly. If the test runs without problems, you can then break past the line in execution that is throwing the exception to view debug information after the line executes.
Also, let us know if the test step worked before you customized the step in code.
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