diff --git a/gtsam/nonlinear/ExpressionFactor.h b/gtsam/nonlinear/ExpressionFactor.h
index c8137a098..8a6f28713 100644
--- a/gtsam/nonlinear/ExpressionFactor.h
+++ b/gtsam/nonlinear/ExpressionFactor.h
@@ -31,6 +31,7 @@ namespace gtsam {
  */
 template<typename T>
 class ExpressionFactor: public NoiseModelFactor {
+  BOOST_CONCEPT_ASSERT((IsTestable<T>));
 
 protected: