HOW abnormal are Swedes, and other people in the Nordic region, in paying tax? A general stereotype for Europe holds some truth: unlike tax-shy southern Europeans, those in the far north pay up readily to get comprehensive, efficient government services—plus societies with unusually equitable income distribution. In Sweden, even after years of slashing high taxes (an inheritance tax went in 2005, another for wealth disappeared in 2007, some taxes on residential property went the next year, corporate tax is low at 22%), the share of GDP claimed by the state remains high. The OECD reckons its government spent over 51% of GDP in 2014. Income tax rates, at least for the well-off, can be as high as 57%. And the Swedes comply. Sociologists, economists and others have long debated this readiness to cough up for the common good. Lutheran beliefs about the importance of supporting the whole community might be a factor. A strong sense of cultural homogeneity for the “folk” matters (now some worry this will be undermined by too much immigration). Maybe the generations spent huddling together to survive long, dark winters has played a role.

This week brought a new puzzle, however, with evidence that some Swedes are deliberately overpaying their taxes. Official figures published on February 22nd showed tax revenues poured in far faster than forecast last year. Sweden’s government got a budget surplus of 85bn kronor ($9.5bn) in 2016. Nearly half of that, 40bn Skr, was from overpayment by firms and individuals. This appears to be deliberate. It also causes a headache for civil servants who will have to make repayments and manage the funds. What is going on?

This is not evidence, in fact, of a new or extreme culture of Nordic eagerness for paying tax. Instead the explanation is financial, a bizarre result of the existence of negative interest rates. For the past two years, since March 2015, Sweden’s central bank has kept rates below 0%, as have other Swedish banks, in a broad effort to have the economy avoid deflation. At the same time, the government promised to pay a positive interest rate—0.56%—for any funds that had been overpaid in tax, upon their return to the taxpayer. Even though this interest payment has since been cut to zero, individuals and companies remain better off storing their savings in the form of overpaid tax, rather watch them shrink in the bank. According to the Financial Times a similar situation, again thanks to an unusually large government surplus, is emerging in Switzerland, which also imposed negative interest rates in 2015.

Officials and politicians would usually crow about a growing budget surplus. But Sweden’s official bean-counters are not cheering. The cost of borrowing from taxpayers, ie taking in their over-payments, is higher than raising funds in other ways. It is also not entirely clear how the practice might grow, nor how quickly those who have overpaid will demand their money back, which all makes it tricky to manage the flow of money. Negative interest rates are likely to continue in Sweden for some time, which suggests that overpayment of taxes will also go on. That sounds like the sort of problem a government in southern Europe would be delighted to have. In Sweden, however, officials may ask taxpayers to cool it and please pay a bit less.