diff --git a/pyopenjtalk/__init__.py b/pyopenjtalk/__init__.py
index 656c508..127f026 100644
--- a/pyopenjtalk/__init__.py
+++ b/pyopenjtalk/__init__.py
@@ -274,12 +274,15 @@ def update_global_jtalk_with_user_dict(path):
     Note that this will change the global state of the openjtalk module.
 
     Args:
-        path (str): path to user dictionary
+        path (str | None): path to user dictionary.
+          If None, Update global openjtalk instance without the user dictionary.
     """
+    if path is None:
+        path = ""
+    elif not exists(path):
+        raise FileNotFoundError("no such file or directory: %s" % path)
     global _global_jtalk
     with _global_jtalk():
-        if not exists(path):
-            raise FileNotFoundError("no such file or directory: %s" % path)
         _global_jtalk = _global_instance_manager(
             instance=OpenJTalk(
                 dn_mecab=OPEN_JTALK_DICT_DIR, userdic=path.encode("utf-8")
diff --git a/tests/test_openjtalk.py b/tests/test_openjtalk.py
index 5a27466..459f37c 100644
--- a/tests/test_openjtalk.py
+++ b/tests/test_openjtalk.py
@@ -89,10 +89,11 @@ def test_g2p_phone():
 
 
 def test_userdic():
-    for text, expected in [
+    parameter = [
         ("nnmn", "n a n a m i N"),
         ("GNU", "g u n u u"),
-    ]:
+    ]
+    for text, expected in parameter:
         p = pyopenjtalk.g2p(text)
         assert p != expected
 
@@ -106,13 +107,16 @@ def test_userdic():
     pyopenjtalk.mecab_dict_index(f.name, user_dic)
     pyopenjtalk.update_global_jtalk_with_user_dict(user_dic)
 
-    for text, expected in [
-        ("nnmn", "n a n a m i N"),
-        ("GNU", "g u n u u"),
-    ]:
+    for text, expected in parameter:
         p = pyopenjtalk.g2p(text)
         assert p == expected
 
+    pyopenjtalk.update_global_jtalk_with_user_dict(None)
+
+    for text, expected in parameter:
+        p = pyopenjtalk.g2p(text)
+        assert p != expected
+
 
 def test_multithreading():
     ojt = pyopenjtalk.openjtalk.OpenJTalk(pyopenjtalk.OPEN_JTALK_DICT_DIR)