fix: default DRILL_LEITNER_BOX to 0 in leitner-rebox

When the property is absent, org-entry-get returns nil and
string-to-number errors with wrong-type-argument. Reachable when a
user removes the property mid-session, or when a Leitner-tagged
entry is rebox'd before its DRILL_LEITNER_BOX has been set.

Wrapped the org-entry-get with (or ... "0"). Box 0 makes the rating
semantics still sensible: a downgrade stays at 0, a promotion goes
to 1.

2 files changed, 49 insertions, 1 deletions

diff --git a/org-drill.el b/org-drill.el
index f98bbfe..5037d73 100644
--- a/org-drill.el
+++ b/org-drill.el
@@ -3986,8 +3986,13 @@ shuffling is done in place."
     (cond
      ((and (>= ch ?0) (<= ch ?5))
       (let ((current-box
+             ;; Default to 0 if the property isn't set — `org-entry-get'
+             ;; returns nil and `string-to-number' would error on nil.
+             ;; Box 0 is sensible: a downgrade stays at 0, a promotion
+             ;; goes to 1.
              (string-to-number
-              (org-entry-get (point) "DRILL_LEITNER_BOX" nil))))
+              (or (org-entry-get (point) "DRILL_LEITNER_BOX" nil)
+                  "0"))))
         (cond
          ((or (= ch ?0))
           (message "Refiled down to box: 1")
diff --git a/tests/test-org-drill-leitner-rebox-missing-box.el b/tests/test-org-drill-leitner-rebox-missing-box.el
new file mode 100644
index 0000000..68c49c9
--- /dev/null
+++ b/tests/test-org-drill-leitner-rebox-missing-box.el
@@ -0,0 +1,43 @@
+;;; test-org-drill-leitner-rebox-missing-box.el --- Regression for missing DRILL_LEITNER_BOX  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; `org-drill-leitner-rebox' read DRILL_LEITNER_BOX via `org-entry-get'
+;; and passed the result straight into `string-to-number'.  When the
+;; property is absent, org-entry-get returns nil and string-to-number
+;; errors with "Wrong type argument: char-or-string-p, nil".  Reachable
+;; if a user removes the property mid-session, or if a Leitner-tagged
+;; entry is rebox'd before its DRILL_LEITNER_BOX is set.
+;;
+;; Fix wraps the value with `(or ... "0")' so a missing property is
+;; treated as box 0 (which makes the rating semantics still sensible —
+;; a "downgrade by one" stays at 0, and a promotion goes to 1).
+
+;;; Code:
+
+(require 'ert)
+(require 'cl-lib)
+(require 'org)
+(require 'org-drill)
+
+;;;; Regression
+
+(ert-deftest test-leitner-rebox-survives-missing-leitner-box-property ()
+  "On a leitner entry without DRILL_LEITNER_BOX, rebox should not crash."
+  (with-temp-buffer
+    (let ((org-startup-folded nil))
+      (insert "* Question :leitner:\nbody\n")    ; no DRILL_LEITNER_BOX
+      (org-mode)
+      (goto-char (point-min))
+      (let ((session (org-drill-session)))
+        (cl-letf (((symbol-function 'read-key-sequence)
+                   (lambda (_prompt) "5"))
+                  ((symbol-function 'sit-for) #'ignore))
+          ;; Should not error.
+          (org-drill-leitner-rebox session)
+          ;; After a quality-5 promotion from missing (treated as box 0):
+          ;; new box = 0 + 1 = 1.  Either way, the property should now exist.
+          (should (org-entry-get (point) "DRILL_LEITNER_BOX")))))))
+
+(provide 'test-org-drill-leitner-rebox-missing-box)
+
+;;; test-org-drill-leitner-rebox-missing-box.el ends here