diff --git a/libevmasm/Assembly.cpp b/libevmasm/Assembly.cpp
index 881107cf2..6fcc17d12 100644
--- a/libevmasm/Assembly.cpp
+++ b/libevmasm/Assembly.cpp
@@ -223,7 +223,7 @@ string Assembly::assemblyString(
 }
 
 Json::Value Assembly::createJsonValue(
-	string _name, int _sourceIndex, size_t _modifierDepth, int _begin, int _end, string _value, string _jumpType)
+	string _name, int _sourceIndex, size_t _modifierDepth, int _begin, int _end, string const& _value, string const& _jumpType)
 {
 	Json::Value value{Json::objectValue};
 	value["name"] = _name;
diff --git a/libevmasm/Assembly.h b/libevmasm/Assembly.h
index 5f0f98abc..3e8002652 100644
--- a/libevmasm/Assembly.h
+++ b/libevmasm/Assembly.h
@@ -191,8 +191,8 @@ private:
 		size_t _modifierDepth,
 		int _begin,
 		int _end,
-		std::string _value = std::string(),
-		std::string _jumpType = std::string()
+		std::string const& _value = std::string(),
+		std::string const& _jumpType = std::string()
 	);
 	static std::string toStringInHex(u256 _value);