diff --git a/instructor/anthropic_utils.py b/instructor/anthropic_utils.py
index ed8ee94..d9fd9f7 100644
--- a/instructor/anthropic_utils.py
+++ b/instructor/anthropic_utils.py
@@ -50,6 +50,7 @@ def _add_params(
     # TODO: handling of nested params with the same name
     properties = model_dict.get("properties", {})
     list_found = False
+    nested_list_found = False
 
     for field_name, details in properties.items():
         parameter = ET.SubElement(root, "parameter")
@@ -74,11 +75,19 @@ def _add_params(
             field_type = details.get(
                 "type", "unknown"
             )  # Might be better to fail here if there is no type since pydantic models require types
+        
+        if "array" in field_type and "items" not in details:
+            raise ValueError("Invalid array item.")
 
-        # Adjust type if array
-        if "array" in field_type or "List" in field_type:
+        # Check for nested List
+        if "array" in field_type and "$ref" in details["items"]:
             type_element.text = f"List[{details['title']}]"
             list_found = True
+            nested_list_found = True     
+        # Check for non-nested List
+        elif "array" in field_type and "type" in details["items"]:
+            type_element.text = f"List[{details['items']['type']}]"
+            list_found = True
         else:
             type_element.text = field_type
 
@@ -105,22 +114,13 @@ def _add_params(
                 reference,
                 references,
             )
-        elif field_type == "array":  # Handling for List[] type
+        elif field_type == "array" and nested_list_found:  # Handling for List[] type
             nested_params = ET.SubElement(parameter, "parameters")
             list_found |= _add_params(
                 nested_params,
                 _resolve_reference(references, details["items"]["$ref"]),
                 references,
             )
-        elif "array" in field_type:  # Handling for optional List[] type
-            nested_params = ET.SubElement(parameter, "parameters")
-            list_found |= _add_params(
-                nested_params,
-                _resolve_reference(
-                    references, details["anyOf"][0]["items"]["$ref"]
-                ),  # CHANGE
-                references,
-            )
 
     return list_found
 
diff --git a/instructor/function_calls.py b/instructor/function_calls.py
index 3e0f9c9..f63040e 100644
--- a/instructor/function_calls.py
+++ b/instructor/function_calls.py
@@ -69,6 +69,7 @@ class OpenAISchema(BaseModel):  # type: ignore[misc]
             for line in parseString(json_to_xml(cls)).toprettyxml().splitlines()[1:]
         )
 
+
     @classmethod
     def from_response(
         cls,
diff --git a/poetry.lock b/poetry.lock
index 259f349..59e0874 100644
--- a/poetry.lock
+++ b/poetry.lock
@@ -1,4 +1,4 @@
-# This file is automatically @generated by Poetry 1.7.1 and should not be changed by hand.
+# This file is automatically @generated by Poetry 1.8.2 and should not be changed by hand.
 
 [[package]]
 name = "aiohttp"
@@ -2677,6 +2677,7 @@ files = [
     {file = "PyYAML-6.0.1-cp311-cp311-win_amd64.whl", hash = "sha256:bf07ee2fef7014951eeb99f56f39c9bb4af143d8aa3c21b1677805985307da34"},
     {file = "PyYAML-6.0.1-cp312-cp312-macosx_10_9_x86_64.whl", hash = "sha256:855fb52b0dc35af121542a76b9a84f8d1cd886ea97c84703eaa6d88e37a2ad28"},
     {file = "PyYAML-6.0.1-cp312-cp312-macosx_11_0_arm64.whl", hash = "sha256:40df9b996c2b73138957fe23a16a4f0ba614f4c0efce1e9406a184b6d07fa3a9"},
+    {file = "PyYAML-6.0.1-cp312-cp312-manylinux_2_17_aarch64.manylinux2014_aarch64.whl", hash = "sha256:a08c6f0fe150303c1c6b71ebcd7213c2858041a7e01975da3a99aed1e7a378ef"},
     {file = "PyYAML-6.0.1-cp312-cp312-manylinux_2_17_x86_64.manylinux2014_x86_64.whl", hash = "sha256:6c22bec3fbe2524cde73d7ada88f6566758a8f7227bfbf93a408a9d86bcc12a0"},
     {file = "PyYAML-6.0.1-cp312-cp312-musllinux_1_1_x86_64.whl", hash = "sha256:8d4e9c88387b0f5c7d5f281e55304de64cf7f9c0021a3525bd3b1c542da3b0e4"},
     {file = "PyYAML-6.0.1-cp312-cp312-win32.whl", hash = "sha256:d483d2cdf104e7c9fa60c544d92981f12ad66a457afae824d146093b8c294c54"},
diff --git a/tests/anthropic/test_simple.py b/tests/anthropic/test_simple.py
index c8cce0a..205306a 100644
--- a/tests/anthropic/test_simple.py
+++ b/tests/anthropic/test_simple.py
@@ -63,6 +63,31 @@ def test_nested_type():
     assert resp.address.street_name == "First Avenue"
 
 
+def test_list():
+    class User(BaseModel):
+        name: str
+        age: int
+        family: List[str]
+    
+    resp = create(
+        model="claude-3-opus-20240229", # Fails with claude-3-haiku-20240307
+        max_tokens=1024,
+        max_retries=0,
+        messages=[
+            {
+                "role": "user",
+                "content": "Create a user for a model with a name, age, and family members.",
+            }
+        ],
+        response_model=User,
+    )
+    
+    assert isinstance(resp, User)
+    assert isinstance(resp.family, List)
+    for member in resp.family:
+        assert isinstance(member, str)
+
+        
 def test_nested_list():
     class Properties(BaseModel):
         key: str