=== modified file 'src/Products/PluginIndexes/KeywordIndex/KeywordIndex.py'
--- src/Products/PluginIndexes/KeywordIndex/KeywordIndex.py	2010-04-01 15:13:15 +0000
+++ src/Products/PluginIndexes/KeywordIndex/KeywordIndex.py	2010-05-15 23:11:57 +0000
@@ -70,7 +70,8 @@
             try:
                 for kw in newKeywords:
                     self.insertForwardIndexEntry(kw, documentId)
-                self._unindex[documentId] = list(newKeywords)
+                if newKeywords:
+                    self._unindex[documentId] = list(newKeywords)
             except TypeError:
                 return 0
         else:
@@ -83,7 +84,10 @@
             rdiff = difference(newKeywords, oldKeywords)
             if fdiff or rdiff:
                 # if we've got forward or reverse changes
-                self._unindex[documentId] = list(newKeywords)
+                if newKeywords:
+                    self._unindex[documentId] = list(newKeywords)
+                else:
+                    del self._unindex[documentId]
                 if fdiff:
                     self.unindex_objectKeywords(documentId, fdiff)
                 if rdiff:

=== modified file 'src/Products/PluginIndexes/interfaces.py'
--- src/Products/PluginIndexes/interfaces.py	2010-04-14 14:02:08 +0000
+++ src/Products/PluginIndexes/interfaces.py	2010-05-15 23:16:41 +0000
@@ -34,10 +34,22 @@
     def index_object(documentId, obj, threshold=None):
         """Index an object.
 
-        'documentId' is the integer ID of the document.
-        'obj' is the object to be indexed.
-        'threshold' is the number of words to process between committing
-        subtransactions.  If None, subtransactions are disabled.
+        - ``documentId`` is the integer ID of the document.
+
+        - ``obj`` is the object to be indexed.
+
+        - ``threshold`` is the number of words to process between committing
+          subtransactions.  If None, subtransactions are disabled.
+
+        For each name in ``getIndexSourceNames``, try to get the named
+        attribute from ``obj``.
+
+        - If the object does not have the attribute, do not add it to the
+          index for that name.
+
+        - If the attribute is a callable, call it to get the value.  If
+          calling it raises an AttributeError, do not add it to the index.
+          for that name.
         """
 
     def unindex_object(documentId):