diff --git a/ua_parser/user_agent_parser.py b/ua_parser/user_agent_parser.py
index 2c3d1bcd..192d25ab 100644
--- a/ua_parser/user_agent_parser.py
+++ b/ua_parser/user_agent_parser.py
@@ -18,6 +18,7 @@
 
 import os
 import re
+import sys
 import warnings
 
 __author__ = "Lindsey Simon <elsigh@gmail.com>"
@@ -218,8 +219,15 @@ def Parse(self, user_agent_string):
 MAX_CACHE_SIZE = 200
 _PARSE_CACHE = {}
 
+_UA_TYPES = str
+if sys.version_info < (3,):
+    _UA_TYPES = (str, unicode)
+
 
 def _lookup(ua, args):
+    if not isinstance(ua, _UA_TYPES):
+        raise TypeError("Expected user agent to be a string, got %r" % ua)
+
     key = (ua, tuple(sorted(args.items())))
     entry = _PARSE_CACHE.get(key)
     if entry is not None:
diff --git a/ua_parser/user_agent_parser_test.py b/ua_parser/user_agent_parser_test.py
index 6658cb39..9d3384c2 100644
--- a/ua_parser/user_agent_parser_test.py
+++ b/ua_parser/user_agent_parser_test.py
@@ -29,6 +29,7 @@
 import os
 import platform
 import re
+import sys
 import unittest
 import warnings
 import yaml
@@ -301,5 +302,26 @@ def test_js_bits_deprecation(self):
                 self.assertEqual(w.category, DeprecationWarning)
 
 
+class ErrTest(unittest.TestCase):
+    @unittest.skipIf(
+        sys.version_info < (3,), "bytes and str are not differentiated in P2"
+    )
+    def test_bytes(self):
+        with self.assertRaises(TypeError):
+            user_agent_parser.Parse(b"")
+
+    def test_int(self):
+        with self.assertRaises(TypeError):
+            user_agent_parser.Parse(0)
+
+    def test_list(self):
+        with self.assertRaises(TypeError):
+            user_agent_parser.Parse([])
+
+    def test_tuple(self):
+        with self.assertRaises(TypeError):
+            user_agent_parser.Parse(())
+
+
 if __name__ == "__main__":
     unittest.main()