diff --git a/gtsam_unstable/nonlinear/ExpressionFactor.h b/gtsam_unstable/nonlinear/ExpressionFactor.h
index f609071cb..22a2aefeb 100644
--- a/gtsam_unstable/nonlinear/ExpressionFactor.h
+++ b/gtsam_unstable/nonlinear/ExpressionFactor.h
@@ -17,6 +17,8 @@
  * @brief Expressions for Block Automatic Differentiation
  */
 
+#pragma once
+
 #include <gtsam_unstable/nonlinear/Expression.h>
 #include <gtsam/nonlinear/NonlinearFactor.h>
 #include <gtsam/base/Testable.h>