A customer pointed out in his blog at http://sudhirvn.blogspot.com/2009/01/adcenter-error-code-1514-negative.html that error code 1514 will be returned when adding negative keywords if the keyword only has an exact match bid. This is correct and the documentation does not explain this.
When setting negative keywords for a keyword, the keyword must have at least one of a broad or phrase match bid. You cannot set negative keywords on a keyword that only has an exact match bid.
The documentation will be updated in the near future. Thanks to Sudhir for pointing out this error.
Programming Writer - adCenterAPI