diff --git a/poincare/src/expression.cpp b/poincare/src/expression.cpp
index ae4d3c1a1..16a0c757f 100644
--- a/poincare/src/expression.cpp
+++ b/poincare/src/expression.cpp
@@ -312,7 +312,9 @@ Expression Expression::addMissingParentheses() {
 }
 
 void Expression::defaultDeepReduceChildren(ExpressionNode::ReductionContext reductionContext) {
-  for (int i = 0; i < numberOfChildren(); i++) {
+  const int childrenCount = numberOfChildren();
+  for (int i = 0; i < childrenCount; i++) {
+    assert(childrenCount == numberOfChildren());
     childAtIndex(i).deepReduce(reductionContext);
   }
 }