'Null' and 'undefined' in bibliography

If I omit the publication title for webpages in my metadata, 'null' or 'undefined' is inserted into the bibliography where the publication would go like this:

Billerman, S. M., Keeney, B. K., Rodewald, P. G. & Schulenberg, T. S. 2020. Birds of the World. undefined. [Retrieved from https://birdsoftheworld.org/bow/home on 20th February 2021].

Is this a metadata issue or a problem with the code in my csl file?
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