diff --git a/src/lighteval/metrics/utils/llm_as_judge.py b/src/lighteval/metrics/utils/llm_as_judge.py
index e18b3a4c0..b5b0b7f0c 100644
--- a/src/lighteval/metrics/utils/llm_as_judge.py
+++ b/src/lighteval/metrics/utils/llm_as_judge.py
@@ -134,7 +134,7 @@ def __init__(
         self.hf_provider = hf_provider
         self.max_tokens = max_tokens
 
-        self.response_format = response_format if not None else DEFAULT_FORMAT
+        self.response_format = response_format if response_format is not None else DEFAULT_FORMAT
 
         self.backend_options = backend_options or {}
 
diff --git a/tests/unit/metrics/test_llm_as_judge_response_format.py b/tests/unit/metrics/test_llm_as_judge_response_format.py
new file mode 100644
index 000000000..87b5a149a
--- /dev/null
+++ b/tests/unit/metrics/test_llm_as_judge_response_format.py
@@ -0,0 +1,58 @@
+# MIT License
+
+# Copyright (c) 2024 The HuggingFace Team
+
+# Permission is hereby granted, free of charge, to any person obtaining a copy
+# of this software and associated documentation files (the "Software"), to deal
+# in the Software without restriction, including without limitation the rights
+# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+# copies of the Software, and to permit persons to whom the Software is
+# furnished to do so, subject to the following conditions:
+
+# The above copyright notice and this permission notice shall be included in all
+# copies or substantial portions of the Software.
+
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+# SOFTWARE.
+
+"""Regression tests for JudgeLM response_format defaulting."""
+
+from pydantic import BaseModel
+
+from lighteval.metrics.utils.llm_as_judge import DEFAULT_FORMAT, JudgeLM
+
+
+def _make_judge(**kwargs):
+    return JudgeLM(
+        model="gpt-4o",
+        templates=lambda **kw: [{"role": "user", "content": "q"}],
+        process_judge_response=lambda x: x,
+        judge_backend="openai",
+        **kwargs,
+    )
+
+
+def test_response_format_defaults_when_not_provided():
+    """When response_format is not given, DEFAULT_FORMAT must be applied.
+
+    The previous expression `response_format if not None else DEFAULT_FORMAT`
+    tested the constant `not None` (always True), so DEFAULT_FORMAT was
+    unreachable and None leaked into the request kwargs of every backend.
+    """
+    judge = _make_judge()
+
+    assert judge.response_format == DEFAULT_FORMAT
+
+
+def test_explicit_response_format_is_preserved():
+    class JudgeVerdict(BaseModel):
+        score: float
+
+    judge = _make_judge(response_format=JudgeVerdict)
+
+    assert judge.response_format is JudgeVerdict