diff --git a/vterm-module.c b/vterm-module.c
index 2294ef4..cc0ef53 100644
--- a/vterm-module.c
+++ b/vterm-module.c
@@ -1149,6 +1149,12 @@ static VTermParserCallbacks parser_callbacks = {
 };
 #else
 
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wbitfield-constant-conversion"
+/* -1 is represented as ~0 in the bit field. */
+static const size_t invalid_string_fragment_len = (VTermStringFragment){.len = (size_t)-1}.len;
+#pragma clang diagnostic pop
+
 static int osc_callback(int cmd, VTermStringFragment frag, void *user) {
   /* osc_callback (OSC = Operating System Command) */
 
@@ -1171,7 +1177,11 @@ static int osc_callback(int cmd, VTermStringFragment frag, void *user) {
     return 0;
   }
 
-  term->cmd_buffer = concat(term->cmd_buffer, frag.str, frag.len, true);
+  /* frag.len can be -1 (invalid) for sequences such as "\033];\033". https://github.com/akermu/emacs-libvterm/issues/729 */
+  /* This might be a bug in libvterm. */
+  if (frag.len != invalid_string_fragment_len) {
+    term->cmd_buffer = concat(term->cmd_buffer, frag.str, frag.len, true);
+  }
   if (frag.final) {
     handle_osc_cmd(term, cmd, term->cmd_buffer);
     free(term->cmd_buffer);