diff --git a/test/evaluation/harness/rag/test_harness.py b/test/evaluation/harness/rag/test_harness.py
index 65e528c2..2918420a 100644
--- a/test/evaluation/harness/rag/test_harness.py
+++ b/test/evaluation/harness/rag/test_harness.py
@@ -1,7 +1,7 @@
 # SPDX-FileCopyrightText: 2022-present deepset GmbH <info@deepset.ai>
 #
 # SPDX-License-Identifier: Apache-2.0
-from typing import Any, Dict, List, Optional
+from typing import Any, Dict, List, Optional, Union
 import pytest
 
 from haystack_experimental.evaluation.harness.rag import (
@@ -106,7 +106,9 @@ def run(self, query: str) -> Dict[str, Any]:
 
 @component
 class MockEvaluator:
-    def __init__(self, metric: RAGEvaluationMetric) -> None:
+    def __init__(self, metric: Union[str, RAGEvaluationMetric]) -> None:
+        if isinstance(metric, str):
+            metric = RAGEvaluationMetric(metric)
         self.metric = metric
 
         io_map = {
@@ -132,6 +134,9 @@ def __init__(self, metric: RAGEvaluationMetric) -> None:
         self.__haystack_input__ = io_map[metric].__haystack_input__
         self.__haystack_output__ = io_map[metric].__haystack_output__
 
+    def to_dict(self):
+        return default_to_dict(self, metric=str(self.metric))
+
     @staticmethod
     def default_output(metric) -> Dict[str, Any]:
         if metric in (