diff --git a/src/authentic2/crypto.py b/src/authentic2/crypto.py
index 3abf9eb81..27f291c05 100644
--- a/src/authentic2/crypto.py
+++ b/src/authentic2/crypto.py
@@ -191,10 +191,10 @@ def aes_base64url_deterministic_decrypt(key, urlencoded, salt, raise_on_error=Tr
 
 
 def hmac_url(key, url):
-    if hasattr(key, 'isnumeric'):
-        key = key.encode('utf-8')
-    if hasattr(url, 'isnumeric'):
-        url = url.encode('utf-8', 'replace')
+    if hasattr(key, 'encode'):
+        key = key.encode()
+    if hasattr(url, 'encode'):
+        url = url.encode()
     return (
         base64.b32encode(hmac.HMAC(key=key, msg=url, digestmod=hashlib.sha256).digest())
         .decode('ascii')
@@ -203,6 +203,6 @@ def hmac_url(key, url):
 
 
 def check_hmac_url(key, url, signature):
-    if hasattr(signature, 'isnumeric'):
-        signature = signature.encode('utf-8', 'replace')
+    if hasattr(signature, 'decode'):
+        signature = signature.decode()
     return constant_time_compare(signature, hmac_url(key, url).encode('ascii'))