diff --git a/requests_http_signature/__init__.py b/requests_http_signature/__init__.py
index fe37e29..b2781a5 100644
--- a/requests_http_signature/__init__.py
+++ b/requests_http_signature/__init__.py
@@ -87,7 +87,7 @@ class HTTPSignatureAuth(requests.auth.AuthBase):
             if header == "(request-target)":
                 path_url = requests.models.RequestEncodingMixin.path_url.fget(request)
                 sts.append("{}: {} {}".format(header, request.method.lower(), path_url))
-            elif header == "(created)":
+            elif header == "(created)" and created_timestamp:
                 sts.append("{}: {}".format(header, created_timestamp))
             elif header == "(expires)":
                 assert (expires_timestamp is not None), \
@@ -157,7 +157,7 @@ class HTTPSignatureAuth(requests.auth.AuthBase):
         for field in "keyId", "algorithm", "signature":
             assert field in sig_struct, 'Required signature parameter "{}" not found'.format(field)
         assert sig_struct["algorithm"] in self.known_algorithms, "Unknown signature algorithm"
-        created_timestamp = int(sig_struct['created'])
+        created_timestamp = int(sig_struct['created']) if 'created' in sig_struct else None
         expires_timestamp = sig_struct.get('expires')
         if expires_timestamp is not None:
             expires_timestamp = int(expires_timestamp)