Resolution:
Correct the code; then recompile.
Example:
The following example shows code that would throw this error.
       class-id a abstract.
       indexer-id string (i as binary-long) abstract.
       getter.
       setter.
       end indexer.
       end class.
       class-id b inherits type a.
       indexer-id string (i as binary-long) override.
       getter.
       end indexer.
       end class.