diff --git a/minio/commonconfig.py b/minio/commonconfig.py
index 993e6967..6e5eaf6a 100644
--- a/minio/commonconfig.py
+++ b/minio/commonconfig.py
@@ -466,12 +466,12 @@ def build_headers(self, object_size: int, etag: str):
     @property
     def object_size(self) -> Optional[int]:
         """Get object size."""
-        if self.object_size is None:
+        if self._object_size is None:
             raise MinioException(
                 "build_headers() must be called prior to "
                 "this method invocation",
             )
-        return self.object_size
+        return self._object_size
 
     @property
     def headers(self) -> dict[str, str]:
diff --git a/tests/functional/tests.py b/tests/functional/tests.py
index 58935976..3b3b6aa5 100644
--- a/tests/functional/tests.py
+++ b/tests/functional/tests.py
@@ -53,10 +53,13 @@
 from minio.time import to_http_header
 from minio.versioningconfig import SUSPENDED, VersioningConfig
 
+# pylint: disable=invalid-name
 _CLIENT = None  # initialized in main().
 _TEST_FILE = None  # initialized in main().
 _LARGE_FILE = None  # initialized in main().
 _IS_AWS = None  # initialized in main().
+# pylint: enable=invalid-name
+
 KB = 1024
 MB = 1024 * KB
 HTTP = urllib3.PoolManager(
diff --git a/tests/unit/composesource_test.py b/tests/unit/composesource_test.py
new file mode 100644
index 00000000..c1114a2c
--- /dev/null
+++ b/tests/unit/composesource_test.py
@@ -0,0 +1,30 @@
+# -*- coding: utf-8 -*-
+# MinIO Python Library for Amazon S3 Compatible Cloud Storage,
+# (C) 2015, 2016 MinIO, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+from unittest import TestCase
+
+from minio.commonconfig import ComposeSource
+from minio.error import MinioException
+
+
+class ComposeSourceTest(TestCase):
+    def test_object_size(self):
+        source = ComposeSource(bucket_name="my-bucket",
+                               object_name="my-object")
+        with self.assertRaises(MinioException) as exc:
+            _ = source.object_size
+
+        msg = "build_headers() must be called prior to this method invocation"
+        self.assertEqual(msg, str(exc.exception))