diff --git a/sagemaker-serve/tests/unit/test_model_builder_register_framework_defaults.py b/sagemaker-serve/tests/unit/test_model_builder_register_framework_defaults.py
new file mode 100644
index 0000000000..57cb7b8c33
--- /dev/null
+++ b/sagemaker-serve/tests/unit/test_model_builder_register_framework_defaults.py
@@ -0,0 +1,116 @@
+# Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License"). You
+# may not use this file except in compliance with the License. A copy of
+# the License is located at
+#
+#     http://aws.amazon.com/apache2.0/
+#
+# or in the "license" file accompanying this file. This file 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.
+"""Regression tests: ModelBuilder.register() must not require the user to set
+``framework`` / ``framework_version`` manually.
+
+GitHub issue #5391: register() raised
+``AttributeError: 'ModelBuilder' object has no attribute 'framework'`` (and the
+same for ``framework_version``) unless the user explicitly assigned them before
+calling register(). These attributes are internal and should be initialized to
+sensible defaults (None) at construction so register() works out of the box.
+
+These tests lock in that a freshly constructed ModelBuilder already exposes both
+attributes defaulted to None, and that register() reaches
+``create_model_package_from_containers`` without an AttributeError when the user
+never touches them.
+"""
+from __future__ import absolute_import
+
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+
+_ROLE = "arn:aws:iam::123456789012:role/SageMakerRole"
+
+
+@pytest.fixture
+def model_builder_cls():
+    """Import ModelBuilder lazily so import issues stay scoped to this fixture."""
+    from sagemaker.serve import ModelBuilder
+
+    return ModelBuilder
+
+
+def _make_builder(model_builder_cls):
+    """Construct a ModelBuilder without ever setting framework attributes.
+
+    The role resolver is patched so construction does no IAM work offline.
+    """
+    session = MagicMock(boto_region_name="us-west-2")
+    # register() is telemetry-wrapped and reads sagemaker_config; a bare
+    # MagicMock value fails config-schema validation, so pin it to an empty dict.
+    session.sagemaker_config = {}
+    with patch(
+        "sagemaker.serve.model_builder.resolve_and_validate_role",
+        return_value=_ROLE,
+    ):
+        return model_builder_cls(
+            model="my_module.MyModelClass",
+            sagemaker_session=session,
+        )
+
+
+def test_framework_attributes_default_to_none_on_construction(model_builder_cls):
+    """A freshly built ModelBuilder exposes framework/framework_version = None.
+
+    This is the root-cause guard for #5391: the attributes must exist without
+    the user assigning them.
+    """
+    mb = _make_builder(model_builder_cls)
+
+    assert hasattr(mb, "framework")
+    assert hasattr(mb, "framework_version")
+    assert mb.framework is None
+    assert mb.framework_version is None
+
+
+def test_register_does_not_require_user_set_framework_attributes(model_builder_cls):
+    """register() must not raise AttributeError for framework/framework_version.
+
+    We run the non-pipeline register() path (MagicMock session is not a
+    PipelineSession) and only assert that the framework attributes flow through
+    to update_container_with_inference_params without the user setting them.
+    """
+    mb = _make_builder(model_builder_cls)
+    # Minimal state register() needs on the non-pipeline, versioned path.
+    mb.image_uri = "123456789012.dkr.ecr.us-west-2.amazonaws.com/img:latest"
+    mb.s3_model_data_url = "s3://bucket/model/"
+    mb.content_types = ["application/json"]
+    mb.response_types = ["application/json"]
+
+    arn = "arn:aws:sagemaker:us-west-2:123456789012:model-package/g/1"
+
+    captured = {}
+
+    def _capture_params(**kwargs):
+        # Record the framework values register() passed through.
+        captured["framework"] = kwargs.get("framework")
+        captured["framework_version"] = kwargs.get("framework_version")
+        return kwargs.get("container_def")
+
+    with patch(
+        "sagemaker.serve.model_builder.create_model_package_from_containers",
+        return_value={"ModelPackageArn": arn},
+    ), patch("sagemaker.serve.model_builder.get_model_package_args", return_value={}), patch(
+        "sagemaker.serve.model_builder.update_container_with_inference_params",
+        side_effect=_capture_params,
+    ), patch.object(
+        mb, "_prepare_container_def", return_value={"Image": mb.image_uri}
+    ):
+        result = mb.register(model_package_group_name="g")
+
+    assert result == arn
+    # framework attributes were read (defaulted to None), not missing.
+    assert captured["framework"] is None
+    assert captured["framework_version"] is None