diff --git a/src/openai/_compat.py b/src/openai/_compat.py
index 020ffeb2ca..286b5be3f7 100644
--- a/src/openai/_compat.py
+++ b/src/openai/_compat.py
@@ -142,19 +142,21 @@ def model_dump(
     by_alias: bool | None = None,
 ) -> dict[str, Any]:
     if (not PYDANTIC_V1) or hasattr(model, "model_dump"):
-        return model.model_dump(
+        kwargs: dict[str, Any] = dict(
             mode=mode,
             exclude=exclude,
             exclude_unset=exclude_unset,
             exclude_defaults=exclude_defaults,
             # warnings are not supported in Pydantic v1
             warnings=True if PYDANTIC_V1 else warnings,
-            by_alias=by_alias,
         )
+        if by_alias is not None:
+            kwargs["by_alias"] = by_alias
+        return model.model_dump(**kwargs)
     return cast(
         "dict[str, Any]",
         model.dict(  # pyright: ignore[reportDeprecated, reportUnnecessaryCast]
-            exclude=exclude, exclude_unset=exclude_unset, exclude_defaults=exclude_defaults, by_alias=bool(by_alias)
+            exclude=exclude, exclude_unset=exclude_unset, exclude_defaults=exclude_defaults, by_alias=bool(by_alias) if by_alias is not None else by_alias
         ),
     )
 
diff --git a/tests/test_models.py b/tests/test_models.py
index 588869ee35..05db6a85b1 100644
--- a/tests/test_models.py
+++ b/tests/test_models.py
@@ -157,6 +157,20 @@ def test_unknown_fields() -> None:
     assert model_dump(m2) == {"foo": "foo", "unknown": {"foo_bar": True}}
 
 
+def test_model_dump_by_alias_none() -> None:
+    """Ensure model_dump does not crash when by_alias defaults to None (GH-2921)."""
+    m = BasicModel.construct(foo="hello")
+    # by_alias=None is the default — must not raise TypeError
+    result = model_dump(m)
+    assert result == {"foo": "hello"}
+    # Explicit by_alias=None must also work
+    result = model_dump(m, by_alias=None)
+    assert result == {"foo": "hello"}
+    # Explicit by_alias=True/False must still be forwarded
+    result = model_dump(m, by_alias=False)
+    assert result == {"foo": "hello"}
+
+
 def test_strict_validation_unknown_fields() -> None:
     class Model(BaseModel):
         foo: str