Heart Rate sometimes shows "??"

[fibby]

I have tested the heart rate reading on my own Versa 2 and it works fine but when I submit it for review, It was rejected and I get this message 

Please find the issues uncovered during testing below: # the HR value cannot be 0/null, use -- or leave it empty. In your case the value is ?? when the sensor cannot detect the pulse.

I thought showing question mark when the pulse cannot be detected is fine. What is the usual convention for this situation ?

[TigerPointe]

I think the message from reviewers says that you should use two dashes instead of two question marks.  An empty string is also allowed, but dashes look better.

[fibby]

Thanks. let me try two dashes. I have got some reviewers allowing the two question mark while some do not, I guess there is no official standard.