diff --git a/src/pyotp/utils.py b/src/pyotp/utils.py
index f816703..d863da2 100644
--- a/src/pyotp/utils.py
+++ b/src/pyotp/utils.py
@@ -1,6 +1,10 @@
 from __future__ import print_function, unicode_literals, division, absolute_import
 
 import unicodedata
+try:
+    from itertools import izip_longest
+except ImportError:
+    from itertools import zip_longest as izip_longest
 
 try:
     from urllib.parse import quote
@@ -73,10 +77,9 @@ def strings_equal(s1, s2):
     except ImportError:
         pass
 
-    if len(s1) != len(s2):
-        return False
-
     differences = 0
-    for c1, c2 in zip(s1, s2):
+    for c1, c2 in izip_longest(s1, s2):
+        if c1 is None or c2 is None:
+            pass
         differences |= ord(c1) ^ ord(c2)
     return differences == 0