mailto link issue in 9.5.5 and 9.6.6

[Scott Randby]

Hello,

I'm having a problem with mailto links. The problem occurs in Org 9.5.5 
(running emacs -q) and Org 9.6.6 at least. It did not occur when I used Org 
9.4.6.

I do not use Emacs for email (sorry). The documentation states that when 
browse-url-mailto-function is set to nil, then the setting of browse-url-browse-function 
will be used. That function is set to browse-url-default-browser in Org 9.5.5, and it is 
set to browse-url-firefox in my Org 9.6.6. However when browse-url-mailto-function is set 
to nil and I C-c C-o on a mailto link, I get this message: "Symbol's definition is 
void: nil"

How do I fix this issue? The other types of links I use (http, https, file, 
info) all work fine.

Scott Randby