diff --git a/poincare/include/poincare/evaluation.h b/poincare/include/poincare/evaluation.h
index e5a426e9f..48c750c26 100644
--- a/poincare/include/poincare/evaluation.h
+++ b/poincare/include/poincare/evaluation.h
@@ -37,7 +37,7 @@ template<typename T>
 class Evaluation : public TreeByValue {
 public:
   EvaluationNode<T> * node() const override {
-    assert(!TreeByValue::node()->isGhost());
+    assert(TreeByValue::node() == nullptr || !TreeByValue::node()->isGhost());
     return static_cast<EvaluationNode<T> *>(TreeByValue::node());
   }
   Evaluation() : Evaluation(nullptr) {}
diff --git a/poincare/include/poincare/expression.h b/poincare/include/poincare/expression.h
index f4625021e..e64b6426b 100644
--- a/poincare/include/poincare/expression.h
+++ b/poincare/include/poincare/expression.h
@@ -34,7 +34,7 @@ public:
 
   /* Reference */
   ExpressionNode * node() const override {
-    assert(!TreeByValue::node()->isGhost());
+    assert(TreeByValue::node() == nullptr || !TreeByValue::node()->isGhost());
     return static_cast<ExpressionNode *>(TreeByValue::node());
   }
 
diff --git a/poincare/include/poincare/layout_reference.h b/poincare/include/poincare/layout_reference.h
index 1717fd312..baf74f221 100644
--- a/poincare/include/poincare/layout_reference.h
+++ b/poincare/include/poincare/layout_reference.h
@@ -29,7 +29,7 @@ public:
   }
 
   LayoutNode * node() const override {
-    assert(!TreeByReference::node()->isGhost());
+    assert(TreeByReference::node() == nullptr || !TreeByReference::node()->isGhost());
     return static_cast<LayoutNode *>(TreeByReference::node());
   }
 
diff --git a/poincare/include/poincare/tree_by_reference.h b/poincare/include/poincare/tree_by_reference.h
index 623a09e14..b4ce0f426 100644
--- a/poincare/include/poincare/tree_by_reference.h
+++ b/poincare/include/poincare/tree_by_reference.h
@@ -117,7 +117,9 @@ protected:
   TreeByReference() : m_identifier(-1) {}
   void setIdentifierAndRetain(int newId) {
     m_identifier = newId;
-    node()->retain();
+    if (isDefined()) {
+      node()->retain();
+    }
   }
   /* Hierarchy operations */
   // Add