diff --git a/packages/sie_server/src/sie_server/api/options.py b/packages/sie_server/src/sie_server/api/options.py
index 9667b3c4f..153ffff33 100644
--- a/packages/sie_server/src/sie_server/api/options.py
+++ b/packages/sie_server/src/sie_server/api/options.py
@@ -53,7 +53,9 @@ def resolve_runtime_options_with_profile(
         ) from e
 
     overflow_policy = merged.get("overflow_policy")
-    if overflow_policy is not None and overflow_policy not in VALID_OVERFLOW_POLICIES:
+    if overflow_policy is not None and (
+        not isinstance(overflow_policy, str) or overflow_policy not in VALID_OVERFLOW_POLICIES
+    ):
         span.set_attribute("error", "invalid_overflow_policy")
         raise HTTPException(
             status_code=status.HTTP_400_BAD_REQUEST,
diff --git a/packages/sie_server/tests/api/test_option.py b/packages/sie_server/tests/api/test_option.py
new file mode 100644
index 000000000..489a03f82
--- /dev/null
+++ b/packages/sie_server/tests/api/test_option.py
@@ -0,0 +1,34 @@
+from unittest.mock import MagicMock
+
+import pytest
+from fastapi import HTTPException
+from sie_server.api.options import resolve_runtime_options_with_profile
+from sie_server.config.model import EmbeddingDim, EncodeTask, ModelConfig, ProfileConfig, Tasks
+
+
+def _make_config() -> ModelConfig:
+    return ModelConfig(
+        sie_id="test",
+        hf_id="org/test",
+        tasks=Tasks(encode=EncodeTask(dense=EmbeddingDim(dim=8))),
+        profiles={
+            "default": ProfileConfig(
+                adapter_path="sie_server.adapters.base:ModelAdapter",
+                max_batch_tokens=8,
+            )
+        },
+    )
+
+
+def test_invalid_overflow_policy_type_returns_400() -> None:
+    span = MagicMock()
+
+    with pytest.raises(HTTPException) as exc_info:
+        resolve_runtime_options_with_profile(
+            _make_config(),
+            {"overflow_policy": ["truncate_text"]},
+            span,
+        )
+
+    assert exc_info.value.status_code == 400
+    assert exc_info.value.detail["code"] == "INVALID_INPUT"