diff --git a/src/vim.h b/src/vim.h
index d320d3a..0447273 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -1398,6 +1398,9 @@
  */
 #define MAXMAPLEN   50
 
+/* Size in bytes of the hash used in the undo file. */
+#define UNDO_HASH_SIZE 32
+
 #ifdef HAVE_FCNTL_H
 # include <fcntl.h>
 #endif