diff --git a/poincare/include/poincare/global_context.h b/poincare/include/poincare/global_context.h
index 3e102cda2..8b5a76b7d 100644
--- a/poincare/include/poincare/global_context.h
+++ b/poincare/include/poincare/global_context.h
@@ -23,6 +23,7 @@ public:
   static constexpr uint16_t k_maxNumberOfMatrixExpressions = 10;
   static Complex * defaultExpression();
 private:
+  static Complex * nanExpression();
   int symbolIndex(const Symbol * symbol) const;
   Expression * m_expressions[k_maxNumberOfScalarExpressions];
   Complex m_pi;
diff --git a/poincare/src/global_context.cpp b/poincare/src/global_context.cpp
index 51819d9bc..89466f31e 100644
--- a/poincare/src/global_context.cpp
+++ b/poincare/src/global_context.cpp
@@ -28,6 +28,11 @@ Complex * GlobalContext::defaultExpression() {
   return defaultExpression;
 }
 
+Complex * GlobalContext::nanExpression() {
+  static Complex * nanExpression = new Complex(Complex::Float(NAN));
+  return nanExpression;
+}
+
 int GlobalContext::symbolIndex(const Symbol * symbol) const {
   int index = symbol->name() - 'A';
   return index;
@@ -42,7 +47,7 @@ const Expression * GlobalContext::expressionForSymbol(const Symbol * symbol) {
   }
   int index = symbolIndex(symbol);
   if (index < 0 || index >= k_maxNumberOfScalarExpressions) {
-    return nullptr;
+    return nanExpression();
   }
   if (m_expressions[index] == nullptr) {
     return defaultExpression();
diff --git a/poincare/src/symbol.cpp b/poincare/src/symbol.cpp
index 2b41192c6..4e21f8aff 100644
--- a/poincare/src/symbol.cpp
+++ b/poincare/src/symbol.cpp
@@ -25,7 +25,7 @@ float Symbol::privateApproximate(Context& context, AngleUnit angleUnit) const {
 
 Expression * Symbol::privateEvaluate(Context& context, AngleUnit angleUnit) const {
   assert(angleUnit != AngleUnit::Default);
-  if (context.expressionForSymbol(this)) {
+  if (context.expressionForSymbol(this) != nullptr) {
     return context.expressionForSymbol(this)->evaluate(context, angleUnit);
   }
   return nullptr;