Full code coverage

I'm trying to obtain full code coverage for the following line of code...

stringWriter.Write(HtmlEncodedString.Format(string.Format("{0,-10:C}", x + y)))

The line above t开发者_如何学运维his one is showing as fully covered and is just writing out a string but this one is only showing as partially covered.

Anybody have any ideas how I can make this line fully covered?

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Are there perhaps return codes or exceptions from either of the Format() methods that your tests do not currently trigger?