After a recent cleanup of treemacs projects, I wanted to add again a known project:

~/src/django

Yet M-x treemacs-add-project returned

(wrong-type-argument hash-table-p nil)

Debugging with M-x debug-on-error gave away a suspicious gethash call:

Debugger entered--Lisp error:
  (wrong-type-argument hash-table-p nil)
  gethash("/home/ra/src/django/crm" nil nil)
  treemacs-add-project("/home/ra/src/django/")
...

gethash("/home/ra/src/django/crm" nil nil) contains crm. But why?

So I started digging in the treemacs options by calling M-x custom-options and found treemacs-persist-file

A short glance at that file gave away the problem:

cat ~/.emacs.default/.cache/treemacs-persist

...
* Default
** crm
 - path :: ~/src/django/crm
...

The project was already there but only it’s subdir (crm) was added to treemacs. Mind that only the parent dir is versioned (.git).

Long story short 💡: