Yields on Germany’s 10-year sovereign bonds plunged below zero for the first time Tuesday as uncertainty surrounding global economic growth and the future of the United Kingdom in the EU fuelled demand for perceived safe havens. The negative yield shows that investors are willing to accept a guaranteed loss in order keep their money safe — effectively paying the German government for the privilege.

At 6:28 a.m. EDT Tuesday, Germany’s 10-year bund yield stood at -0.013. The bund now joins Japan’s and Switzerland’s 10-year sovereign bonds in having yields of less than zero.

According to a Commerzbank estimate cited by the Wall Street Journal, roughly two-thirds of outstanding German sovereign debt now yields less than 0.4 percent. Analysts at Commerzbank also expect the 15-year and 20-year bond yields to tumble below zero in the coming weeks, especially if the U.K. chooses to leave the EU.

“Nobody buys bunds at these yield levels thinking they are attractive,” Jussi Hiljanen, head of European macro- and fixed-income strategy at the Swedish financial group SEB, told Bloomberg. “Demand for haven assets is being driven by fear of Brexit and growth concern. Investors are buying bunds as a hedge against uncertainty.”

Most recent polls show that the Brexit referendum, scheduled for June 23, is still too close to call.

Another factor impacting bond yields is the European Central Bank’s massive quantitative easing program and its negative interest rate regime — measures that are aimed at boosting growth in the eurozone by lowering financing costs.

However, a representative for the German Federal Debt Agency reportedly said that the tradability of the bonds is “still very high.”

“The federal debt-management strategy is long-term, therefore, the current absolute yield level plays only a subordinate role. Our target remains a sustainable balance between cost and planning security for the debt portfolio,” the German Federal Debt Agency told CNBC.