Other worries are more practical. Some Danish financial firms have discovered that their computer systems literally cannot cope with negative rates, and have had to be reprogrammed. The tax code also assumes that rates are always positive.
http://www.economist.com/news/finance-and-economics/21644203...

"tax code" can have other meanings, but in this context it means "the rules/methods used to calculate the amount of tax due"



--------------------------------------------------
Note added at 11 hrs (2015-05-06 09:23:08 GMT)
--------------------------------------------------

IOW when making tax rules no one ever considered the possibility that you could get paid for borrowing money [instead of having to pay interest], so there are simply no calculation methods/rules available to apply, and the system is stuck.