Description
Due to a problem in the Quartus® II software version 13.1 and earlier, you may see this error message if you use a variable as an index for signal.
For example :
data(a downto b) where a and b are variables
Resolution
To workaround this problem, use a 3rd party synthesis tool or avoid this code construct.
This problem is scheduled to be fixed in a future release of the Quartus II software.