diff --git a/src/cloudflare/_models.py b/src/cloudflare/_models.py
index 4fd83e1340d..d08376c2199 100644
--- a/src/cloudflare/_models.py
+++ b/src/cloudflare/_models.py
@@ -335,6 +335,8 @@ def model_dump(
                 raise ValueError("fallback is only supported in Pydantic v2")
             if exclude_computed_fields != False:
                 raise ValueError("exclude_computed_fields is only supported in Pydantic v2")
+            if polymorphic_serialization is not None:
+                raise ValueError("polymorphic_serialization is only supported in Pydantic v2")
             dumped = super().dict(  # pyright: ignore[reportDeprecated]
                 include=include,
                 exclude=exclude,
@@ -398,6 +400,8 @@ def model_dump_json(
                 raise ValueError("ensure_ascii is only supported in Pydantic v2")
             if exclude_computed_fields != False:
                 raise ValueError("exclude_computed_fields is only supported in Pydantic v2")
+            if polymorphic_serialization is not None:
+                raise ValueError("polymorphic_serialization is only supported in Pydantic v2")
             return super().json(  # type: ignore[reportDeprecated]
                 indent=indent,
                 include=include,
diff --git a/tests/test_models.py b/tests/test_models.py
index 73f4ab99dd6..b8c038d2c5a 100644
--- a/tests/test_models.py
+++ b/tests/test_models.py
@@ -537,6 +537,21 @@ class Model2(BaseModel):
             m.to_dict(warnings=False)
 
 
+def test_polymorphic_serialization_is_rejected_on_pydantic_v1() -> None:
+    class Model(BaseModel):
+        foo: int = 1
+
+    m = Model()
+    if PYDANTIC_V1:
+        with pytest.raises(ValueError, match="polymorphic_serialization is only supported in Pydantic v2"):
+            m.model_dump(polymorphic_serialization=True)
+        with pytest.raises(ValueError, match="polymorphic_serialization is only supported in Pydantic v2"):
+            m.model_dump_json(polymorphic_serialization=True)
+
+    # the default must keep working on both major versions
+    assert m.model_dump() == {"foo": 1}
+
+
 def test_forwards_compat_model_dump_method() -> None:
     class Model(BaseModel):
         foo: Optional[str] = Field(alias="FOO", default=None)