Average time computation just prints out "10??"

[David Karr]

I was able to get throughput numbers for benchmarks, but now I want to see
average times for them.  They are pretty short blocks, so it would be nice
if I could get them rendered in microseconds, but I don't see an obvious
way to do that.

However, even before trying to configure the units, I'm trying to
understand the output I'm getting.  The results I get all say "10?? s/op".
I thought perhaps this was trying to render unicode, but when I stored the
output in a file and inspected it, I saw that those really are "?"
characters.

What can I do about this?