Idiomatic C#: when to return null and when to return NaN
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When specifying and creating numeric functions are there any C# idiomatic guidelines on when to return null and when to return NaN when both seem valid outputs.

The concrete example that has lead to this question is I am creating a percentage extension method for Enumerable collections.  I have been thinking about what to return if there is an empty input.  The function could return runningCount / totalCount which would return NaN on zero length inputs.  However, I am not convinced that this sits well with the general feel of the language.

Are there any guidelines or language examples could inform the decision whether to retun NaN or null.