Description
Due to a problem in the Quartus® II software version 9.1 and later, you may see this error when opening your project or during compilation on a Windows computer. This error can occur if your Quartus II database is corrupted.
Resolution
To work around this problem, close the Quartus II software, delete your db and incremental_db directories and then relaunch the Quartus II software to recompile your design.