diff --git a/gtsam/nonlinear/ISAM2-impl.cpp b/gtsam/nonlinear/ISAM2-impl.cpp
index d84740516..3c373e0ba 100644
--- a/gtsam/nonlinear/ISAM2-impl.cpp
+++ b/gtsam/nonlinear/ISAM2-impl.cpp
@@ -200,7 +200,7 @@ void ISAM2::Impl::ExpmapMasked(Values& values, const VectorValues& delta, const
   // Invalidating means setting expmapped entries to Inf, to trigger assertions
   // if we try to re-use them.
 #ifdef NDEBUG
-  invalidateIfDebug = boost::optional<Permuted<VectorValues>&>();
+  invalidateIfDebug = boost::none;
 #endif
 
   assert(values.size() == ordering.nVars());