Messages From Unencrypted SNS Topic Not Reaching Encrypted SQS Queue - amazon-web-services

Question

I have an unencrypted SNS topic with an encrypted SQS queue subscribed to it. Whenever I publish messages from the SNS topic, the SQS queue does not receive the message (the SQS queue receives messages successfully from the SNS topic when encryption is disabled). So far I have been following this guide. Here is what I have completed so far:
Create KMS customer managed key (CMK) with the following key policy ("Configure KMS permissions for AWS services" section in linked guide):
{
"Version": "2012-10-17",
"Id": "key-consolepolicy-3",
"Statement": [
{
"Sid": "Allow administration of the key",
"Effect": "Allow",
"Principal": {
"AWS": "arn:aws:iam::${aws_account_id}:root"
},
"Action": "kms:*",
"Resource": "*"
},
{
"Sid": "Allow SNS to use KMS",
"Effect": "Allow",
"Principal": {
"Service": "sns.amazonaws.com"
},
"Action": [
"kms:GenerateDataKey",
"kms:Decrypt"
],
"Resource": "*"
}
]
}
Currently I am stuck on the "Configure KMS permissions for producers" section. When I add the following statement in the Statement list of my KMS CMK's key policy, I get the error MalformedPolicyDocumentException - Policy contains a statement with no principal.
{
"Effect": "Allow",
"Action": [
"kms:GenerateDataKey",
"kms:Decrypt"
],
"Resource": "${kms_customer_managed_key_arn}"
},
{
"Effect": "Allow",
"Action": [
"sqs:SendMessage"
],
"Resource": "${sqs_queue_arn}"
}
The guide does not include a principal in its policy, so I do not understand why I am getting an error. Can anyone give me any help or advice on how to move forward with this?