worklog.el wrong type argument hash-table-p, nil

[Mark Mynsted]

Recently, to my surprise I receive the following error when I attempt
to execute worklog-summarize-tasks on my .worklog.  (This used to work
fine, e.g. it worked fine last week and I have not altered worklog.el
or emacs.)

wrong type argument hash-table-p, nil

What may I do to fix this?

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