diff --git a/yubico_client/yubico.py b/yubico_client/yubico.py
index <HASH>..<HASH> 100644
--- a/yubico_client/yubico.py
+++ b/yubico_client/yubico.py
@@ -82,7 +82,7 @@ class Yubico(object):
                              ' argument'))
 
         self.client_id = client_id
-        self.key = base64.b64decode(key) if key is not None else None
+        self.key = base64.b64decode(key.encode('ascii')) if key is not None else None
         self.verify_cert = verify_cert
         self.translate_otp = translate_otp
         self.api_urls = self._init_request_urls(api_urls=api_urls)
yubico.py: Convert key to byte string before passing it to b<I>decode

In Python3 <= <I>, the base<I>.b<I>decode function only accepts byte strings, so
we need to convert the key to one.