diff --git a/httpx/_client.py b/httpx/_client.py
index 13cd9336..2de79845 100644
--- a/httpx/_client.py
+++ b/httpx/_client.py
@@ -363,6 +363,16 @@ class BaseClient:
 
         [0]: /advanced/clients/#request-instances
         """
+        # Validate data parameter for better error messages
+        if data is not None and isinstance(data, list):
+            # Check if this looks like invalid JSON array (list of dicts/strings)
+            # but allow valid multipart form data (list of 2-item tuples)
+            if data and all(isinstance(item, (dict, str, int, float, bool)) for item in data):
+                raise TypeError(
+                    "Invalid value for 'data'. To send a JSON array, use the 'json' parameter. "
+                    "For form data, use a dictionary or a list of 2-item tuples."
+                )
+
         url = self._merge_url(url)
         headers = self._merge_headers(headers)
         cookies = self._merge_cookies(cookies)
diff --git a/tests/client/test_async_client.py b/tests/client/test_async_client.py
index 8d7eaa3c..91b7065c 100644
--- a/tests/client/test_async_client.py
+++ b/tests/client/test_async_client.py
@@ -373,3 +373,43 @@ async def test_server_extensions(server):
         response = await client.get(url)
     assert response.status_code == 200
     assert response.extensions["http_version"] == b"HTTP/1.1"
+
+
+INVALID_DATA_FORMATS_ASYNC = [
+    pytest.param([{"a": "b"}], id="list-of-dicts"),
+    pytest.param(["a", "b", "c"], id="list-of-strings"),
+    pytest.param([1, 2, 3], id="list-of-integers"),
+]
+
+
+@pytest.mark.anyio
+@pytest.mark.parametrize("invalid_data", INVALID_DATA_FORMATS_ASYNC)
+async def test_async_build_request_with_invalid_data_list(invalid_data):
+    """
+    Verify that AsyncClient.build_request raises a helpful TypeError for invalid list formats.
+    """
+    async with httpx.AsyncClient() as client:
+        expected_message = (
+            "Invalid value for 'data'. To send a JSON array, use the 'json' parameter. "
+            "For form data, use a dictionary or a list of 2-item tuples."
+        )
+        with pytest.raises(TypeError, match=expected_message):
+            client.build_request("POST", "https://example.com", data=invalid_data)
+
+
+@pytest.mark.anyio
+async def test_async_build_request_with_valid_data_formats():
+    """
+    Verify that AsyncClient.build_request accepts valid data formats without raising our custom TypeError.
+    """
+    async with httpx.AsyncClient() as client:
+        # Test with a dictionary
+        request = client.build_request("POST", "https://example.com", data={"a": "b"})
+        assert isinstance(request, httpx.Request)
+
+        # Test with a list of 2-item tuples (for multipart)
+        # This is a valid use case and should not raise our TypeError.
+        # We explicitly catch and ignore the DeprecationWarning that httpx raises in this specific case.
+        with pytest.warns(DeprecationWarning):
+            request = client.build_request("POST", "https://example.com", data=[("a", "b")])
+        assert isinstance(request, httpx.Request)
diff --git a/tests/client/test_client.py b/tests/client/test_client.py
index 65783901..fa96f7cb 100644
--- a/tests/client/test_client.py
+++ b/tests/client/test_client.py
@@ -460,3 +460,42 @@ def test_client_decode_text_using_explicit_encoding():
         assert response.reason_phrase == "OK"
         assert response.encoding == "ISO-8859-1"
         assert response.text == text
+
+
+INVALID_DATA_FORMATS_SYNC = [
+    pytest.param([{"a": "b"}], id="list-of-dicts"),
+    pytest.param(["a", "b", "c"], id="list-of-strings"),
+    pytest.param([1, 2, 3], id="list-of-integers"),
+]
+
+
+@pytest.mark.parametrize("invalid_data", INVALID_DATA_FORMATS_SYNC)
+def test_sync_build_request_with_invalid_data_list(invalid_data):
+    """
+    Verify that Client.build_request raises a helpful TypeError for invalid list formats.
+    """
+    client = httpx.Client()
+    expected_message = (
+        "Invalid value for 'data'. To send a JSON array, use the 'json' parameter. "
+        "For form data, use a dictionary or a list of 2-item tuples."
+    )
+    with pytest.raises(TypeError, match=expected_message):
+        client.build_request("POST", "https://example.com", data=invalid_data)
+
+
+def test_sync_build_request_with_valid_data_formats():
+    """
+    Verify that Client.build_request accepts valid data formats without raising our custom TypeError.
+    """
+    client = httpx.Client()
+
+    # Test with a dictionary
+    request = client.build_request("POST", "https://example.com", data={"a": "b"})
+    assert isinstance(request, httpx.Request)
+
+    # Test with a list of 2-item tuples (for multipart)
+    # This is a valid use case and should not raise our TypeError.
+    # We explicitly catch and ignore the DeprecationWarning that httpx raises in this specific case.
+    with pytest.warns(DeprecationWarning):
+        request = client.build_request("POST", "https://example.com", data=[("a", "b")])
+    assert isinstance(request, httpx.Request)