The European Central Bank may have to tweak the rules of its massive bond buying programme because German debt is in short supply.

The ECB plans to buy €60 billion ($65 billion) of assets each month until at least September 2016 to kick-start the region's economy. German government bonds form a cornerstone of the programme, accounting for about €11 billion in monthly purchases. The ECB has said it would buy bonds from two to 30 years in length as long as the yield is above its deposit rate of -0.2%, and that it doesn't expect any problems finding them.