diff --git a/gtsam/linear/HessianFactor.cpp b/gtsam/linear/HessianFactor.cpp
index 970051faf..c631d8d14 100644
--- a/gtsam/linear/HessianFactor.cpp
+++ b/gtsam/linear/HessianFactor.cpp
@@ -392,10 +392,11 @@ void HessianFactor::updateATA(const HessianFactor& update, const Scatter& scatte
 
   // Apply updates to the upper triangle
   gttic(update);
+  size_t nrInfoBlocks = this->info_.nBlocks();
   for(DenseIndex j2=0; j2<update.info_.nBlocks(); ++j2) {
-    DenseIndex slot2 = (j2 == (DenseIndex)update.size()) ? this->info_.nBlocks()-1 : slots[j2];
+    DenseIndex slot2 = (j2 == (DenseIndex)update.size()) ? nrInfoBlocks-1 : slots[j2];
     for(DenseIndex j1=0; j1<=j2; ++j1) {
-      DenseIndex slot1 = (j1 == (DenseIndex)update.size()) ? this->info_.nBlocks()-1 : slots[j1];
+      DenseIndex slot1 = (j1 == (DenseIndex)update.size()) ? nrInfoBlocks-1 : slots[j1];
       if(slot2 > slot1)
         info_(slot1, slot2).noalias() += update.info_(j1, j2);
       else if(slot1 > slot2)
@@ -450,10 +451,13 @@ void HessianFactor::updateATA(const JacobianFactor& update, const Scatter& scatt
 
     // Apply updates to the upper triangle
     gttic(update);
-    for(DenseIndex j2=0; j2<updateBlocks.nBlocks(); ++j2) { // Horizontal block of Hessian
-      DenseIndex slot2 = (j2 == update.size()) ? this->info_.nBlocks()-1 : slots[j2];
+    size_t nrInfoBlocks = this->info_.nBlocks(), nrUpdateBlocks = updateBlocks.nBlocks();
+    for(DenseIndex j2=0; j2<nrUpdateBlocks; ++j2) { // Horizontal block of Hessian
+      DenseIndex slot2 = (j2 == update.size()) ? nrInfoBlocks-1 : slots[j2];
+      assert(slot2>=0 && slot2<=nrInfoBlocks);
       for(DenseIndex j1=0; j1<=j2; ++j1) { // Vertical block of Hessian
-        DenseIndex slot1 = (j1 == update.size()) ? this->info_.nBlocks()-1 : slots[j1];
+        DenseIndex slot1 = (j1 == update.size()) ? nrInfoBlocks-1 : slots[j1];
+        assert(slot1>=0 && slot1<nrInfoBlocks);
         updateBlocks.offset(0);
         if(slot2 > slot1)
           info_(slot1, slot2).noalias() += updateBlocks(j1).transpose() * updateBlocks(j2);