diff --git a/src/c++/perf_analyzer/genai-perf/genai_perf/llm_inputs/synthetic_image_generator.py b/src/c++/perf_analyzer/genai-perf/genai_perf/llm_inputs/synthetic_image_generator.py
index c3e76eb72..f8e778ed4 100644
--- a/src/c++/perf_analyzer/genai-perf/genai_perf/llm_inputs/synthetic_image_generator.py
+++ b/src/c++/perf_analyzer/genai-perf/genai_perf/llm_inputs/synthetic_image_generator.py
@@ -2,7 +2,7 @@
 from enum import Enum, auto
 from io import BytesIO
 from pathlib import Path
-from typing import Optional, Tuple, cast
+from typing import List, Optional, Tuple, cast
 
 import numpy as np
 from genai_perf.exceptions import GenAIPerfException
@@ -15,7 +15,7 @@ class ImageFormat(Enum):
 
 
 class RandomFormatBase64Encoder:
-    def __init__(self, image_formats: ImageFormat = ImageFormat.PNG):
+    def __init__(self, image_formats: List[ImageFormat] = [ImageFormat.PNG]):
         self.image_formats = image_formats
 
     def __call__(self, image):