Made reading/writing undo info a bit more robust.
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index 6d0ce24..c6d0b40 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -8478,7 +8478,7 @@
     char_u hash[UNDO_HASH_SIZE];
 
     u_compute_hash(hash);
-    u_read_undo(eap->arg, hash);
+    u_read_undo(eap->arg, hash, NULL);
 }
 #endif