diff --git a/moto/acm/responses.py b/moto/acm/responses.py
index 7bf12bbb8..431a8cf60 100644
--- a/moto/acm/responses.py
+++ b/moto/acm/responses.py
@@ -185,7 +185,7 @@ class AWSCertificateManagerResponse(BaseResponse):
         idempotency_token = self._get_param('IdempotencyToken')
         subject_alt_names = self._get_param('SubjectAlternativeNames')
 
-        if len(subject_alt_names) > 10:
+        if subject_alt_names is not None and len(subject_alt_names) > 10:
             # There is initial AWS limit of 10
             msg = 'An ACM limit has been exceeded. Need to request SAN limit to be raised'
             return json.dumps({'__type': 'LimitExceededException', 'message': msg}), dict(status=400)