diff --git a/moto/acm/models.py b/moto/acm/models.py
index 39be8945d..15a1bd44d 100644
--- a/moto/acm/models.py
+++ b/moto/acm/models.py
@@ -243,7 +243,7 @@ class CertBundle(BaseModel):
                 'KeyAlgorithm': key_algo,
                 'NotAfter': datetime_to_epoch(self._cert.not_valid_after),
                 'NotBefore': datetime_to_epoch(self._cert.not_valid_before),
-                'Serial': self._cert.serial,
+                'Serial': self._cert.serial_number,
                 'SignatureAlgorithm': self._cert.signature_algorithm_oid._name.upper().replace('ENCRYPTION', ''),
                 'Status': self.status,  # One of PENDING_VALIDATION, ISSUED, INACTIVE, EXPIRED, VALIDATION_TIMED_OUT, REVOKED, FAILED.
                 'Subject': 'CN={0}'.format(self.common_name),
diff --git a/setup.py b/setup.py
index 8ac06c4dd..99be632db 100755
--- a/setup.py
+++ b/setup.py
@@ -37,7 +37,7 @@ install_requires = [
     "jsondiff==1.1.2",
     "aws-xray-sdk!=0.96,>=0.93",
     "responses>=0.9.0",
-    "idna<2.8,>=2.5",
+    "idna<2.9,>=2.5",
     "cfn-lint",
 ]