diff --git a/libsolidity/codegen/YulUtilFunctions.cpp b/libsolidity/codegen/YulUtilFunctions.cpp
index 8e54e417d..14dcc201c 100644
--- a/libsolidity/codegen/YulUtilFunctions.cpp
+++ b/libsolidity/codegen/YulUtilFunctions.cpp
@@ -922,7 +922,7 @@ string YulUtilFunctions::readFromCalldata(Type const& _type)
 	return readFromMemoryOrCalldata(_type, true);
 }
 
-string YulUtilFunctions::updateStorageValueFunction(Type const& _type, boost::optional<unsigned> const _offset)
+string YulUtilFunctions::updateStorageValueFunction(Type const& _type, boost::optional<unsigned> const& _offset)
 {
 	string const functionName =
 		"update_storage_value_" +
diff --git a/libsolidity/codegen/YulUtilFunctions.h b/libsolidity/codegen/YulUtilFunctions.h
index 04228d5bb..ae41edd70 100644
--- a/libsolidity/codegen/YulUtilFunctions.h
+++ b/libsolidity/codegen/YulUtilFunctions.h
@@ -193,7 +193,7 @@ public:
 	/// the specified slot and offset. If offset is not given, it is expected as
 	/// runtime parameter.
 	/// signature: (slot, [offset,] value)
-	std::string updateStorageValueFunction(Type const& _type, boost::optional<unsigned> const _offset = boost::optional<unsigned>());
+	std::string updateStorageValueFunction(Type const& _type, boost::optional<unsigned> const& _offset = boost::optional<unsigned>());
 
 	/// Returns the name of a function that will write the given value to
 	/// the specified address.