problems setting ess-source-directory

I have had the following lines in my emacs init.el file for years:

(setq ess-source-directory
       (lambda ()
          (concat ess-directory "src/")))

This had been taken from ESS documentation from long ago and still 
appears in the current documentation.  But now it causes a problem.  ESS 
loads without problem in emacs.  However, when I attempt to dump and 
edit an existing function (using C-c C-e C-d) while in a running R 
process buffer, I get the following message in the mini-buffer and the 
dump doesn't occur:

Wrong type argument: sequencep, ess-r-customize-alist

This is frustrating and I've had to resort to dumping files in a 
fixed-name sub-directory under my home directory.

ESS version:  20250110.1437 (25.1.0)
Emacs version:  29.4
R version:  4.4.2 (2024-10-31)
Fedora 41 Workstation

Thanks for considering this issue,
Bill McCoy

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