diff --git a/poincare/src/store.cpp b/poincare/src/store.cpp
index 3e3d296ff..8697eeecc 100644
--- a/poincare/src/store.cpp
+++ b/poincare/src/store.cpp
@@ -48,10 +48,11 @@ Expression Store::shallowReduce(Context & context, Preferences::AngleUnit angleU
   Expression finalValue;
   if (symbol().type() == ExpressionNode::Type::Function) {
     // In tata + 2 ->f(tata), replace tata with xUnknown symbol
+    assert(symbol().childAtIndex(0).type() == ExpressionNode::Type::Symbol);
     Expression userDefinedUnknown = symbol().childAtIndex(0);
     const char x[2] = {Symbol::SpecialSymbols::UnknownX, 0};
     Symbol xUnknown = Symbol(x, 1);
-    finalValue = childAtIndex(0).replaceSymbolWithExpression(userDefinedUnknown, xUnknown);
+    finalValue = childAtIndex(0).replaceSymbolWithExpression(static_cast<Symbol &>(userDefinedUnknown), xUnknown);
   } else {
     finalValue = childAtIndex(0);
   }