diff --git a/lib/woothee/__init__.py b/lib/woothee/__init__.py
index 600e7b9..eb4a6bc 100644
--- a/lib/woothee/__init__.py
+++ b/lib/woothee/__init__.py
@@ -28,14 +28,14 @@ def parse(useragent):
 
 
 def is_crawler(useragent):
-    return len(useragent) > 0 and useragent != '-'\
+    return useragent is not None and len(useragent) > 0 and useragent != '-'\
         and try_crawler(useragent, {})
 
 
 def exec_parse(useragent):
     result = {}
 
-    if len(useragent) < 1 or useragent == '-':
+    if useragent is None or len(useragent) < 1 or useragent == '-':
         return result
 
     if try_crawler(useragent, result):
diff --git a/tests/test.py b/tests/test.py
index 4d3022c..a7eba44 100644
--- a/tests/test.py
+++ b/tests/test.py
@@ -6,12 +6,10 @@
 import os
 import sys
 import unittest
+import yaml
 
 BASE_PATH = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
 sys.path.insert(0, os.path.join(BASE_PATH, 'lib'))
-
-import yaml
-
 TESTSET_DIR = os.path.join(BASE_PATH, 'woothee', 'testsets')
 
 TARGETS = [
@@ -31,6 +29,7 @@
     ['pc_lowpriority.yaml', 'PC/LowPriority'],
     ['misc.yaml', 'Misc'],
     ['crawler_nonmajor.yaml', 'Crawler/NonMajor'],
+    ['blank.yaml', 'Blank'],
 ]
 
 
@@ -101,16 +100,6 @@ def test_non_provide_testsets(self):
 
         import woothee
 
-        # 33 line lib/woothee/__init__.py
-        self.assertEqual({
-            "name": "UNKNOWN",
-            "version": "UNKNOWN",
-            "os": "UNKNOWN",
-            "os_version": "UNKNOWN",
-            "category": "UNKNOWN",
-            "vendor": "UNKNOWN",
-        }, woothee.parse(""))
-
         # 48 line in lib/woothee/appliance.py
         self.assertEqual({
             "name": "Nintendo DSi",
@@ -268,6 +257,7 @@ def _callFUT(self, *args, **kwargs):
     def test_false(self):
         self.assertFalse(self._callFUT(""))
         self.assertFalse(self._callFUT("-"))
+        self.assertFalse(self._callFUT(None))
         self.assertFalse(self._callFUT(
             "Mozilla/5.0 (Windows NT 6.3; "
             "Trident/7.0; rv:11.0) like Gecko"
diff --git a/woothee b/woothee
index 61766c8..a03d10a 160000
--- a/woothee
+++ b/woothee
@@ -1 +1 @@
-Subproject commit 61766c87f52a1fd866509b06a55b93186b5c4921
+Subproject commit a03d10a77d5aaea9b8e5a0a3ba5accaa71720d78