diff --git a/poincare/include/poincare/expression.h b/poincare/include/poincare/expression.h
index de8a4f236..443166bfa 100644
--- a/poincare/include/poincare/expression.h
+++ b/poincare/include/poincare/expression.h
@@ -135,7 +135,7 @@ public:
   static void simplifyAndBeautify(Expression ** expressionAddress);
   Expression * simplify();
   // TODO: should be virtual pure
-  virtual Expression * immediateSimplify() { return this; };// = 0;
+  virtual Expression * immediateSimplify();
   virtual Expression * immediateBeautify() { return this; };
 
   /* Evaluation Engine
diff --git a/poincare/src/expression.cpp b/poincare/src/expression.cpp
index cbe0dc0c9..6fc4b07b8 100644
--- a/poincare/src/expression.cpp
+++ b/poincare/src/expression.cpp
@@ -6,6 +6,7 @@
 #include <poincare/list_data.h>
 #include <poincare/matrix_data.h>
 #include <poincare/evaluation.h>
+#include <poincare/undefined.h>
 #include <cmath>
 #include "expression_parser.hpp"
 #include "expression_lexer.hpp"
@@ -78,6 +79,7 @@ public:
   }
   Expression * clone() const override { return nullptr; }
   Type type() const override { return Expression::Type::Undefined; }
+  Expression * immediateSimplify() override { return this; }
   ExpressionLayout * privateCreateLayout(FloatDisplayMode floatDisplayMode, ComplexFormat complexFormat) const override {
     return nullptr;
   }
@@ -112,6 +114,15 @@ void Expression::beautify(Expression ** expressionAddress) {
   }
 }
 
+Expression * Expression::immediateSimplify() {
+  for (int i = 0; i< numberOfOperands(); i++) {
+    if (operand(i)->type() == Type::Undefined) {
+      return replaceWith(new Undefined(), true);
+    }
+  }
+  return this;
+}
+
 bool Expression::hasAncestor(const Expression * e) const {
   assert(m_parent != this);
   if (m_parent == e) {
diff --git a/poincare/src/power.cpp b/poincare/src/power.cpp
index a25f69cec..8f7384015 100644
--- a/poincare/src/power.cpp
+++ b/poincare/src/power.cpp
@@ -218,7 +218,7 @@ Expression * Power::immediateSimplify() {
       Multiplication * m = new Multiplication(multOperands, 2, false);
       simplifyRationalRationalPower(p1, static_cast<Rational *>((Expression *)p1->operand(0)), static_cast<Rational *>((Expression *)(p1->operand(1)->operand(0))));
       replaceWith(m, true);
-      return immediateSimplify();
+      return m->immediateSimplify();
     }
   }
   return this;