When I moved forward in the book to the "Custom Data Types" section Mr. Vieira has us download the code from this site, so I did. At that point I decided to examine the ExampleAggregate code to see if it would display the same error, and alas it worked just fine. On comparison between the code in the sample download and that printed in the book I found the problem.
On page 324 is a line that I thought looked strange when I read it, and then even more strange when I typed it into the C# struct. That line reads:
SqlContext.Pipe.Send("WARNING: Aggregate values exist and were ignored");
You see, the odd thing is there is nothing in the function to determine that condition. Plus, there probably is something that has to do with checking on the data table which from the error message appears to be prohibited by this type of assembly. Anyway, that line is not present in the downloaded code, which is why it runs as expected.