diff --git a/libyul/backends/wasm/WasmAST.h b/libyul/backends/wasm/WasmAST.h
index 02862aea2..635b7c4ea 100644
--- a/libyul/backends/wasm/WasmAST.h
+++ b/libyul/backends/wasm/WasmAST.h
@@ -25,6 +25,7 @@
 #include <vector>
 #include <map>
 #include <memory>
+#include <optional>
 
 namespace solidity::yul::wasm
 {
@@ -76,7 +77,7 @@ struct FunctionImport {
 	std::string externalName;
 	std::string internalName;
 	std::vector<std::string> paramTypes;
-	std::unique_ptr<std::string> returnType;
+	std::optional<std::string> returnType;
 };
 
 struct FunctionDefinition
diff --git a/libyul/backends/wasm/WasmCodeTransform.cpp b/libyul/backends/wasm/WasmCodeTransform.cpp
index 9c553b380..ea88efbf7 100644
--- a/libyul/backends/wasm/WasmCodeTransform.cpp
+++ b/libyul/backends/wasm/WasmCodeTransform.cpp
@@ -29,6 +29,8 @@
 
 #include <liblangutil/Exceptions.h>
 
+#include <optional>
+
 using namespace std;
 using namespace solidity;
 using namespace solidity::yul;
@@ -125,7 +127,7 @@ wasm::Expression WasmCodeTransform::operator()(FunctionCall const& _call)
 					builtin->name.str().substr(4),
 					builtin->name.str(),
 					{},
-					builtin->returns.empty() ? nullptr : make_unique<string>(builtin->returns.front().str())
+					builtin->returns.empty() ? nullopt : make_optional<string>(builtin->returns.front().str())
 				};
 				for (auto const& param: builtin->parameters)
 					imp.paramTypes.emplace_back(param.str());