diff --git a/mitogen/core.py b/mitogen/core.py
index 1634b031..d2d20723 100644
--- a/mitogen/core.py
+++ b/mitogen/core.py
@@ -420,7 +420,7 @@ class Importer(object):
             'utils',
         ]}
         self._lock = threading.Lock()
-        self.whitelist = whitelist or ['']
+        self.whitelist = list(whitelist) or ['']
         self.blacklist = list(blacklist) + [
             # 2.x generates needless imports for 'builtins', while 3.x does the
             # same for '__builtin__'. The correct one is built-in, the other
diff --git a/tests/importer_test.py b/tests/importer_test.py
index 9d533752..55cad182 100644
--- a/tests/importer_test.py
+++ b/tests/importer_test.py
@@ -32,6 +32,8 @@ class ImporterBlacklist(testlib.TestCase):
         importer = mitogen.core.Importer(
             router=mock.Mock(), context=None, core_src='',
         )
+        self.assertIsInstance(importer.whitelist, list)
+        self.assertIsInstance(importer.blacklist, list)
         self.assertFalse(mitogen.core.is_blacklisted_import(importer, 'mypkg'))
         self.assertFalse(mitogen.core.is_blacklisted_import(importer, 'mypkg.mod'))
         self.assertFalse(mitogen.core.is_blacklisted_import(importer, 'otherpkg'))
@@ -44,6 +46,8 @@ class ImporterBlacklist(testlib.TestCase):
             router=mock.Mock(), context=None, core_src='',
             whitelist=('mypkg',),
         )
+        self.assertIsInstance(importer.whitelist, list)
+        self.assertIsInstance(importer.blacklist, list)
         self.assertFalse(mitogen.core.is_blacklisted_import(importer, 'mypkg'))
         self.assertFalse(mitogen.core.is_blacklisted_import(importer, 'mypkg.mod'))
         self.assertTrue(mitogen.core.is_blacklisted_import(importer, 'otherpkg'))
@@ -56,6 +60,8 @@ class ImporterBlacklist(testlib.TestCase):
             router=mock.Mock(), context=None, core_src='',
             blacklist=('mypkg',),
         )
+        self.assertIsInstance(importer.whitelist, list)
+        self.assertIsInstance(importer.blacklist, list)
         self.assertTrue(mitogen.core.is_blacklisted_import(importer, 'mypkg'))
         self.assertTrue(mitogen.core.is_blacklisted_import(importer, 'mypkg.mod'))
         self.assertFalse(mitogen.core.is_blacklisted_import(importer, 'otherpkg'))
@@ -68,6 +74,8 @@ class ImporterBlacklist(testlib.TestCase):
             router=mock.Mock(), context=None, core_src='',
             whitelist=('mypkg',), blacklist=('mypkg',),
         )
+        self.assertIsInstance(importer.whitelist, list)
+        self.assertIsInstance(importer.blacklist, list)
         self.assertTrue(mitogen.core.is_blacklisted_import(importer, 'mypkg'))
         self.assertTrue(mitogen.core.is_blacklisted_import(importer, 'mypkg.mod'))
         self.assertTrue(mitogen.core.is_blacklisted_import(importer, 'otherpkg'))