diff --git a/lisp/org-src.el b/lisp/org-src.el
index 20e0b598c..e1f7d50dc 100644
--- a/lisp/org-src.el
+++ b/lisp/org-src.el
@@ -479,6 +479,8 @@ (defun org-src--contents-for-write-back (write-back-buf)
                            (buffer-substring eol (point-max))))))
 	(write-back org-src--allow-write-back)
         marker indent-str)
+    ;; Compute the exact sequence of tabs and spaces used to indent up
+    ;; to `indentation-offset' in the Org buffer.
     (setq indent-str
           (with-temp-buffer
             ;; Reproduce indentation parameters from org buffer.
@@ -500,6 +502,9 @@ (defun org-src--contents-for-write-back (write-back-buf)
         ;; first line.
         (when preserve-fl (forward-line))
         (while (not (eobp))
+          ;; Keep empty src lines empty, even when src block is
+          ;; indented on Org side.
+          ;; See https://list.orgmode.org/725763.1632663635@apollo2.minshall.org/T/
           (when (not (eolp)) (insert indent-str)) ; not an empty line
 	  (forward-line)))
       (set-marker marker nil))))