How can I delete topic index keywords that are no longer associated with a topic?

I am trying to clean up the index of an older Robohelp project before it becomes part of a merged webhelp project. I do not want to include any of its index keywords or topic keywords. I've managed to remove most of them, but I have a group of troublesome entries that refuse to go away. In fact, they cause RH8 to crash. I am running Windows XP.

Looking at the code in Visual Studio, it appears that an instruction is pointing to field which contains "zero".  (see image below) 

I have tried creating a new index and deleting the old index, but RH just keeps adding these topic keywords (in bold typeface) to the new index. If the keywords are no longer associated with a topic, where is RH storing these disassociated keywords? If I open the .hhk, it is empty.