diff --git a/python/gtsam/preamble/nonlinear.h b/python/gtsam/preamble/nonlinear.h
index 34b864027..b50059a2e 100644
--- a/python/gtsam/preamble/nonlinear.h
+++ b/python/gtsam/preamble/nonlinear.h
@@ -10,4 +10,6 @@
  * Without this they will be automatically converted to a Python object, and all
  * mutations on Python side will not be reflected on C++.
  */
-#include <pybind11/stl.h>
\ No newline at end of file
+#include <pybind11/stl.h>
+
+PYBIND11_MAKE_OPAQUE(std::vector<size_t>);