How to add a missing value into a double/integr column?

I am implementing a node. It result has some integer/double columns. But in some case there will be a cell without any value. I try to add a string cell with "?" into the columns just like the file reader dose. But when I run the workflow it complains "Runtime class of Object ? is StringCell and does not comply with its supposed super class DoubleCell".

Is there anyway to add a "?" cell into a double columns?

Thank you.

OK, I found.