Question type that supports a text box and "I don't know" answer

Hi,
The survey is gathering technical data, and many times, the responder does not have the data, so we would want the responder to select "I don't know" versus leaving it blank (we cannot assume blank = I don't know).
For example, do you know how many technical bugs in the latest release?
option 1 text box for the responder to enter the number
option 2 select I don't know