Unit tests not running when fork set to "class"

I have a number of unit tests that rely on being run in a separate JVM as they are in Maven or Ant.  When I set the fork option in IDEA to "class," it instantiates all the unit test classes, then says "Done: 0 of 106 (8.537 s)"  The list of unit tests is displayed in the Run window, but none have been executed.  I also get a message in the lower left corner that reads "Failed to start: 0 passed, 106 not started."  Any advice would be greatly appreciated.  I've tried a number of things and can't seem to get the tests to run.  Thanks!