diff --git a/openfeature/provider/_registry.py b/openfeature/provider/_registry.py
index 7529d9fa..d3656c60 100644
--- a/openfeature/provider/_registry.py
+++ b/openfeature/provider/_registry.py
@@ -24,6 +24,8 @@ def __init__(self) -> None:
     def set_provider(self, domain: str, provider: FeatureProvider) -> None:
         if provider is None:
             raise GeneralError(error_message="No provider")
+        if domain is None:
+            raise GeneralError(error_message="No domain")
         providers = self._providers
         if domain in providers:
             old_provider = providers[domain]
diff --git a/tests/provider/test_registry.py b/tests/provider/test_registry.py
new file mode 100644
index 00000000..22613329
--- /dev/null
+++ b/tests/provider/test_registry.py
@@ -0,0 +1,107 @@
+from unittest.mock import Mock
+
+import pytest
+
+from openfeature.exception import GeneralError
+from openfeature.provider._registry import ProviderRegistry
+from openfeature.provider.no_op_provider import NoOpProvider
+
+
+def test_registry_serves_noop_as_default():
+    registry = ProviderRegistry()
+
+    assert isinstance(registry.get_default_provider(), NoOpProvider)
+    assert isinstance(registry.get_provider("unknown domain"), NoOpProvider)
+
+
+def test_setting_provider_requires_domain():
+    registry = ProviderRegistry()
+
+    with pytest.raises(GeneralError) as exc_info:
+        registry.set_provider(None, NoOpProvider())  # type: ignore[reportArgumentType]
+
+    assert exc_info.value.error_message == "No domain"
+
+
+def test_setting_provider_requires_provider():
+    registry = ProviderRegistry()
+
+    with pytest.raises(GeneralError) as exc_info:
+        registry.set_provider("domain", None)  # type: ignore[reportArgumentType]
+
+    assert exc_info.value.error_message == "No provider"
+
+
+def test_can_register_provider_to_multiple_domains():
+    registry = ProviderRegistry()
+    provider = NoOpProvider()
+
+    registry.set_provider("domain1", provider)
+    registry.set_provider("domain2", provider)
+
+    assert registry.get_provider("domain1") is provider
+    assert registry.get_provider("domain2") is provider
+
+
+def test_registering_provider_replaces_previous_provider():
+    """Test that registering a provider replaces the previous provider and calls shutdown on the old one."""
+
+    registry = ProviderRegistry()
+    provider1 = Mock()
+    provider2 = Mock()
+
+    registry.set_provider("domain", provider1)
+    assert registry.get_provider("domain") is provider1
+
+    registry.set_provider("domain", provider2)
+    assert registry.get_provider("domain") is provider2
+
+    provider1.shutdown.assert_called_once()
+    provider2.shutdown.assert_not_called()
+
+
+def test_registering_provider_for_first_time_initializes_it():
+    """Test that registering a provider for the first time calls its initialize method."""
+
+    registry = ProviderRegistry()
+    provider = Mock()
+
+    registry.set_provider("domain1", provider)
+    registry.set_provider("domain2", provider)
+
+    provider.initialize.assert_called_once()
+
+
+def test_setting_default_provider_requires_provider():
+    registry = ProviderRegistry()
+
+    with pytest.raises(GeneralError) as exc_info:
+        registry.set_default_provider(None)  # type: ignore[reportArgumentType]
+
+    assert exc_info.value.error_message == "No provider"
+
+
+def test_replacing_default_provider_shuts_down_old_one():
+    """Test that replacing the default provider shuts down the old default provider."""
+
+    registry = ProviderRegistry()
+    default_provider1 = Mock()
+    default_provider2 = Mock()
+
+    registry.set_default_provider(default_provider1)
+    assert registry.get_default_provider() is default_provider1
+
+    registry.set_default_provider(default_provider2)
+    assert registry.get_default_provider() is default_provider2
+
+    default_provider1.shutdown.assert_called_once()
+    default_provider2.shutdown.assert_not_called()
+
+
+def test_setting_default_provider_initializes_it():
+    registry = ProviderRegistry()
+    provider = Mock()
+
+    registry.set_default_provider(provider)
+
+    provider.initialize.assert_called_once()