diff --git a/acm/README.md b/acm/README.md
index f20f23b..9dbac16 100644
--- a/acm/README.md
+++ b/acm/README.md
@@ -5,6 +5,9 @@ This module creates and ACM certificate, using the general purpose (ca1) ACM-PCA
 include the DNS name in the SAN.  You may add additonal SAN fully qualified domain names, but only DNS names are supported
 in the SAN for an ACM certificate.
 
+It expects an SSM parameter `/enterprise/pki/ca1` to exist in the account (distributed to all OUs from a central account).
+If this parameter does not exist, this module will fail.
+
 It returns the ACM ARN.
 
 # Usage
diff --git a/acm/main.tf b/acm/main.tf
index 5be2f68..8320156 100644
--- a/acm/main.tf
+++ b/acm/main.tf
@@ -5,6 +5,9 @@
 * include the DNS name in the SAN.  You may add additonal SAN fully qualified domain names, but only DNS names are supported
 * in the SAN for an ACM certificate.
 * 
+* It expects an SSM parameter `/enterprise/pki/ca1` to exist in the account (distributed to all OUs from a central account).
+* If this parameter does not exist, this module will fail.
+* 
 * It returns the ACM ARN.
 *
 * # Usage