diff --git a/gtsam/slam/SmartFactorBase.h b/gtsam/slam/SmartFactorBase.h
index 80dde3563..e0540cc41 100644
--- a/gtsam/slam/SmartFactorBase.h
+++ b/gtsam/slam/SmartFactorBase.h
@@ -243,7 +243,7 @@ protected:
    * to the matrices and vectors that will be used to store the results instead
    * of pointers.
    */
-  template<class POINT, class ...OptArgs>
+  template<class POINT, class ...OptArgs, typename = std::enable_if_t<sizeof...(OptArgs)!=0>>
   Vector unwhitenedError(
       const Cameras& cameras, const POINT& point,
       OptArgs&&... optArgs) const {