diff --git a/jedi/inference/compiled/access.py b/jedi/inference/compiled/access.py
index f47ae773..f76a645f 100644
--- a/jedi/inference/compiled/access.py
+++ b/jedi/inference/compiled/access.py
@@ -570,4 +570,6 @@ def _is_class_instance(obj):
     except AttributeError:
         return False
     else:
-        return cls != type and not issubclass(cls, NOT_CLASS_TYPES)
+        # The isinstance check for cls is just there so issubclass doesn't
+        # raise an exception.
+        return cls != type and isinstance(cls, type) and not issubclass(cls, NOT_CLASS_TYPES)
diff --git a/test/test_api/test_interpreter.py b/test/test_api/test_interpreter.py
index 160639eb..1ec0b741 100644
--- a/test/test_api/test_interpreter.py
+++ b/test/test_api/test_interpreter.py
@@ -530,6 +530,16 @@ def test__wrapped__():
     assert c.line == syslogs_to_df.__wrapped__.__code__.co_firstlineno + 1
 
 
+@pytest.mark.skipif(sys.version_info[0] == 2, reason="Ignore Python 2, because EOL")
+def test_illegal_class_instance():
+    class X:
+        __class__ = 1
+    X.__name__ = 'asdf'
+    d, = jedi.Interpreter('foo', [{'foo': X()}]).infer()
+    v, = d._name.infer()
+    assert not v.is_instance()
+
+
 @pytest.mark.skipif(sys.version_info[0] == 2, reason="Ignore Python 2, because EOL")
 @pytest.mark.parametrize('module_name', ['sys', 'time', 'unittest.mock'])
 def test_core_module_completes(module_name):