diff --git a/src/mbyte.c b/src/mbyte.c
index 7a3a27e..cf70f86 100644
--- a/src/mbyte.c
+++ b/src/mbyte.c
@@ -1855,6 +1855,7 @@
  * Convert the character at screen position "off" to a sequence of bytes.
  * Includes the composing characters.
  * "buf" must at least have the length MB_MAXBYTES.
+ * Only to be used when ScreenLinesUC[off] != 0.
  * Returns the produced number of bytes.
  */
     int