diff --git a/src/fileio.c b/src/fileio.c
index df694234fa..a3113d7b19 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -7060,8 +7060,23 @@ buf_check_timestamp(buf, focus)
     }
 
     if (reload)
+    {
 	/* Reload the buffer. */
 	buf_reload(buf, orig_mode);
+#ifdef FEAT_PERSISTENT_UNDO
+	if (buf->b_p_udf && buf->b_ffname != NULL)
+	{
+	    char_u	    hash[UNDO_HASH_SIZE];
+	    buf_T	    *save_curbuf = curbuf;
+
+	    /* Any existing undo file is unusable, write it now. */
+	    curbuf = buf;
+	    u_compute_hash(hash);
+	    u_write_undo(NULL, FALSE, buf, hash);
+	    curbuf = save_curbuf;
+	}
+#endif
+    }
 
 #ifdef FEAT_AUTOCMD
     /* Trigger FileChangedShell when the file was changed in any way. */
diff --git a/src/version.c b/src/version.c
index f30551b867..2f20fa674d 100644
--- a/src/version.c
+++ b/src/version.c
@@ -719,6 +719,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    646,
 /**/
     645,
 /**/