diff --git a/poincare/src/expression.cpp b/poincare/src/expression.cpp
index 3b303c09f..9e1008cf0 100644
--- a/poincare/src/expression.cpp
+++ b/poincare/src/expression.cpp
@@ -126,18 +126,19 @@ void Expression::Simplify(Expression ** expressionAddress, Context & context, An
     angleUnit = Preferences::sharedPreferences()->angleUnit();
   }
   SimplificationRoot root(*expressionAddress);
-  root.deepReduce(context, angleUnit);
-  root.deepBeautify(context, angleUnit);
+  root.editableOperand(0)->deepReduce(context, angleUnit);
+  root.editableOperand(0)->deepBeautify(context, angleUnit);
   *expressionAddress = root.editableOperand(0);
 }
 
 void Expression::Reduce(Expression ** expressionAddress, Context & context, AngleUnit angleUnit) {
   SimplificationRoot root(*expressionAddress);
-  root.deepReduce(context, angleUnit);
+  root.editableOperand(0)->deepReduce(context, angleUnit);
   *expressionAddress = root.editableOperand(0);
 }
 
 Expression * Expression::deepReduce(Context & context, AngleUnit angleUnit) {
+  assert(parent() != nullptr);
   for (int i = 0; i < numberOfOperands(); i++) {
     if ((editableOperand(i))->deepReduce(context, angleUnit)->type() == Type::Undefined && this->type() != Type::SimplificationRoot) {
       return replaceWith(new Undefined(), true);
@@ -147,6 +148,7 @@ Expression * Expression::deepReduce(Context & context, AngleUnit angleUnit) {
 }
 
 Expression * Expression::deepBeautify(Context & context, AngleUnit angleUnit) {
+  assert(parent() != nullptr);
   Expression * e = shallowBeautify(context, angleUnit);
   for (int i = 0; i < e->numberOfOperands(); i++) {
     e->editableOperand(i)->deepBeautify(context, angleUnit);
diff --git a/poincare/src/hierarchy.cpp b/poincare/src/hierarchy.cpp
index f8f6b1426..395ce7091 100644
--- a/poincare/src/hierarchy.cpp
+++ b/poincare/src/hierarchy.cpp
@@ -44,7 +44,7 @@ void Hierarchy::replaceOperand(const Expression * oldOperand, Expression * newOp
   Expression ** op = const_cast<Expression **>(operands());
   for (int i=0; i<numberOfOperands(); i++) {
     if (op[i] == oldOperand) {
-      if (oldOperand != nullptr) {
+      if (oldOperand != nullptr && oldOperand->parent() == this) {
         const_cast<Expression *>(oldOperand)->setParent(nullptr);
       }
       if (deleteOldOperand) {