diff --git a/mypy-zope.ini b/mypy-zope.ini
index 3a6cd6b..3a0ed67 100644
--- a/mypy-zope.ini
+++ b/mypy-zope.ini
@@ -1,3 +1,4 @@
 [mypy]
 namespace_packages=True
+warn_unreachable=True
 plugins=mypy_zope:plugin
diff --git a/tests/samples/dmr_unreachable.py b/tests/samples/dmr_unreachable.py
new file mode 100644
index 0000000..36f2b13
--- /dev/null
+++ b/tests/samples/dmr_unreachable.py
@@ -0,0 +1,31 @@
+from typing import Optional
+
+from zope.interface import Interface, implementer
+
+
+class IFoo(Interface):
+    pass
+
+
+@implementer(IFoo)
+class BaseFoo:
+    pass
+
+
+class ChildFoo(BaseFoo):
+    pass
+
+
+class IFooFactory(Interface):
+    def build() -> Optional[IFoo]:
+        pass
+
+
+def build_and_use_foo(client_factory: IFooFactory) -> None:
+    client_protocol = client_factory.build()
+    assert isinstance(client_protocol, ChildFoo)
+    print("Hello")
+
+
+"""
+"""
\ No newline at end of file
diff --git a/tests/samples/isinstance_impl.py b/tests/samples/isinstance_impl.py
new file mode 100644
index 0000000..242744d
--- /dev/null
+++ b/tests/samples/isinstance_impl.py
@@ -0,0 +1,29 @@
+from typing import Optional
+from zope.interface import implementer, Interface
+
+
+class IFoo(Interface):
+    ...
+
+
+@implementer(IFoo)
+class MyFoo:
+    ...
+
+
+def make_foo() -> Optional[IFoo]:
+    return MyFoo()
+
+
+x = make_foo()
+reveal_type(x)
+assert isinstance(x, MyFoo)
+
+# The code below should not be considered unreachable
+print("hello")
+
+"""
+<output>
+isinstance_impl.py:19: note: Revealed type is "Union[__main__.IFoo, None]"
+</output>
+"""
diff --git a/tests/test_samples.py b/tests/test_samples.py
index f853009..6111e7e 100644
--- a/tests/test_samples.py
+++ b/tests/test_samples.py
@@ -26,6 +26,7 @@ def test_samples(samplefile, mypy_cache_dir):
     opts.show_traceback = True
     opts.namespace_packages = True
     opts.hide_error_codes = True
+    opts.warn_unreachable = True
     opts.plugins = ['mypy_zope:plugin']
     # Config file is needed to load plugins, it doesn't not exist and is not
     # supposed to.