diff --git a/src/misc2.c b/src/misc2.c
index d176303..75d8a4d 100644
--- a/src/misc2.c
+++ b/src/misc2.c
@@ -1366,6 +1366,7 @@
 
 /*
  * Like strncpy(), but always terminate the result with one NUL.
+ * "to" must be "len + 1" long!
  */
     void
 vim_strncpy(to, from, len)