If you are targeting a Virtex design, and you have instantiated an output cell with a non-LVTTL voltage standard, Foundation Express v2.1.2 will produce an error in the optimization phase of synthesis.
Foundation Express 1.5 will report during optimize:
Internal Error: Invalid value '' for attribute 'pmap_slew' on ...... (FE-INTERNAL-padmap-2).
This issue has been resolved with Foundation Express 2.1.3. Please upgrade to Foundation F1.5i or newer.