When negative queries are used objects that are not bibtex entries get passed the the 'entries' function to methods that render bibliography entries. These @string and @preface objects cause errors when functions expect to find certain fields in them.
I have written a test to reproduce this case and one way of ensuring the entries function only returns an array of entries.
Add failing test for bibtex string replacement with negative queries.
Fix entries returing non-BibTeX::Entry objects.
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