1 files changed, 9 insertions, 3 deletions

diff --git a/src/alloc.c b/src/alloc.c
index 9b7da1d0f5b..9c63f8fe132 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -1077,15 +1077,21 @@ Garbage collection happens automatically if you cons more than\n\
   tem = Fnthcdr (make_number (30), Vcommand_history);
   if (CONSP (tem))
     XCONS (tem)->cdr = Qnil;
+
   /* Likewise for undo information.  */
   {
     register struct buffer *nextb = all_buffers;
 
     while (nextb)
       {
-        nextb->undo_list 
-          = truncate_undo_list (nextb->undo_list, undo_threshold,
-                                undo_high_threshold);
+        /* If a buffer's undo list is Qt, that means that undo is
+           turned off in that buffer.  Calling truncate_undo_list on
+           Qt tends to return NULL, which effectively turns undo back on.
+           So don't call truncate_undo_list if undo_list is Qt.  */
+        if (! EQ (nextb->undo_list, Qt))
+          nextb->undo_list 
+            = truncate_undo_list (nextb->undo_list, undo_threshold,
+                                  undo_high_threshold);
         nextb = nextb->next;
       }
   }