I propose the following solution for this problem.
** Patch added: "Proposed patch"
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Negative search terms don't cope with NULL columns
Status in Zeitgeist Framework:
The subject origin (and soon the event origin) columns may be NULL
when an event/subject doesn't have a value for the field. However, the
negation/wildcard handling code doesn't cope with this case.
This can be reproduced with the test case
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