constructed entry file path in org-capture-templates

Question

I have just started to use org-mode and I am tweaking an existing configuration to my needs.

In that configuration, the code is itself in an org file and the org-capture-templates is splited in a lot of individual blocks (because there are some text explanations between) like

(add-to-list 'org-capture-templates
             '("j" "Journal entry"
               entry
               (file+datetree "~/org/journal.org")
               "* %?"
               :empty-lines 1 
               ))

Given that I have defined my org-directory to "~/org" and that this path is duplicated in quite every entry definition, in case I would move the org file in say a share or a git repo, I tried to replace the file paths by a construct like

(add-to-list 'org-capture-templates
             '("j" "Journal entry"
               entry
               (file+datetree (concat org-directory "/journal.org"))
               "* %?"
               :empty-lines 1 
               ))

Unfortunately when I try to capture a journal entry I get this

org-capture-expand-file: Invalid file location: nil

while (concat org-directory "/journal.org") evaluates to the same value "~/org/journal.org". I suppose that the concat is missing some quoting, but I did no find the correct form.