diff --git a/diskcache/fanout.py b/diskcache/fanout.py
index 9822ee4..bb48f1e 100644
--- a/diskcache/fanout.py
+++ b/diskcache/fanout.py
@@ -63,7 +63,10 @@ def directory(self):
     def __getattr__(self, name):
         safe_names = {'timeout', 'disk'}
         valid_name = name in DEFAULT_SETTINGS or name in safe_names
-        assert valid_name, 'cannot access {} in cache shard'.format(name)
+        if not valid_name:
+            raise AttributeError(
+                'cannot access {} in cache shard'.format(name)
+            )
         return getattr(self._shards[0], name)
 
     @cl.contextmanager
diff --git a/tests/test_fanout.py b/tests/test_fanout.py
index af221b6..9b38a05 100644
--- a/tests/test_fanout.py
+++ b/tests/test_fanout.py
@@ -45,6 +45,16 @@ def test_init(cache):
     cache.check()
 
 
+def test_getattr_invalid_name_raises_attribute_error(cache):
+    # Regression test: __getattr__ used to raise AssertionError for an
+    # unknown attribute, which breaks hasattr() (it only treats
+    # AttributeError as "attribute not found").
+    with pytest.raises(AttributeError):
+        cache.__slots__
+
+    assert not hasattr(cache, '__slots__')
+
+
 def test_init_path(cache):
     path = pathlib.Path(cache.directory)
     other = dc.FanoutCache(path)