diff --git a/src/buffer.c b/src/buffer.c
index a2bff81ebd8..16321df850d 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -3879,6 +3879,8 @@ A string is printed verbatim in the mode line except for %-constructs:\n\
 	For a modified read-only buffer, %* gives % and %+ gives *.\n\
   %s -- print process status.   %l -- print the current line number.\n\
   %c -- print the current column number (this makes editing slower).\n\
+        To make the column number update correctly in all cases,\n\
+	`column-number-mode' must be non-nil.\n\
   %p -- print percent of buffer above top of window, or Top, Bot or All.\n\
   %P -- print percent of buffer above bottom of window, perhaps plus Top,\n\
         or print Bottom or All.\n\