diff --git a/poincare/src/serialization_helper.cpp b/poincare/src/serialization_helper.cpp
index c4392478e..5df53c590 100644
--- a/poincare/src/serialization_helper.cpp
+++ b/poincare/src/serialization_helper.cpp
@@ -63,7 +63,7 @@ int SerializationHelper::Infix(
       return bufferSize-1;
     }
     // Write the child, with parentheses if needed
-    serializeChild(interface->serializableChildAtIndex(firstChildIndex), interface, buffer, bufferSize, floatDisplayMode, numberOfDigits, &numberOfChar);
+    serializeChild(interface->serializableChildAtIndex(i), interface, buffer, bufferSize, floatDisplayMode, numberOfDigits, &numberOfChar);
     if (numberOfChar >= bufferSize-1) {
       return bufferSize-1;
     }