Error sending MIME Multipart message through AWS Lambda via Amazon SES - amazon-web-services

Question

I am trying to send an email using Amazon SES, AWS S3, and AWS Lambda together. I have been hitting an error like this for awhile now and I am not completely sure what to do here. I have the stack trace from the error below.
Edit: I have a fully verified Amazon SES domain and I am receiving emails to trigger the Lambda function. I am also able to successfully send emails using the built-in testing features, just not using this function.
{
"errorMessage": "'list' object has no attribute 'encode'",
"errorType": "AttributeError",
"stackTrace": [
" File \"/var/task/lambda_function.py\", line 222, in lambda_handler\n message = create_message(file_dict, header_from, header_to)\n",
" File \"/var/task/lambda_function.py\", line 175, in create_message\n \"Data\": msg.as_string()\n",
" File \"/var/lang/lib/python3.7/email/message.py\", line 158, in as_string\n g.flatten(self, unixfrom=unixfrom)\n",
" File \"/var/lang/lib/python3.7/email/generator.py\", line 116, in flatten\n self._write(msg)\n",
" File \"/var/lang/lib/python3.7/email/generator.py\", line 195, in _write\n self._write_headers(msg)\n",
" File \"/var/lang/lib/python3.7/email/generator.py\", line 222, in _write_headers\n self.write(self.policy.fold(h, v))\n",
" File \"/var/lang/lib/python3.7/email/_policybase.py\", line 326, in fold\n return self._fold(name, value, sanitize=True)\n",
" File \"/var/lang/lib/python3.7/email/_policybase.py\", line 369, in _fold\n parts.append(h.encode(linesep=self.linesep, maxlinelen=maxlinelen))\n"
]
}
Additionally, here is the relevant code. The start of the code is within a create_message() method
import os
import boto3
import email
import re
from botocore.exceptions import ClientError
from email.mime.multipart import MIMEMultipart
from email.mime.text import MIMEText
from email.mime.application import MIMEApplication
. . .
# Create a MIME container.
msg = MIMEMultipart()
# Create a MIME text part.
text_part = MIMEText(body_text, _subtype="html")
# Attach the text part to the MIME message.
msg.attach(text_part)
# Add subject, from and to lines.
msg['Subject'] = subject
msg['From'] = sender
msg['To'] = recipient
# Create a new MIME object.
att = MIMEApplication(file_dict["file"], filename)
att.add_header("Content-Disposition", 'attachment', filename=filename)
# Attach the file object to the message.
msg.attach(att)
message = {
"Source": sender,
"Destinations": recipient,
"Data": msg.as_string() # The error occurs here
}
return message
def send_email(message):
aws_region = os.environ['Region']
# Create a new SES client.
client_ses = boto3.client('ses', region)
# Send the email.
try:
#Provide the contents of the email.
response = client_ses.send_raw_email(
Source=message['Source'],
Destinations=[
message['Destinations']
],
RawMessage={
'Data':message['Data']
}
)
If you have any insight as far as what should be done, that would be greatly appreciated. I've looked at similar questions but they did resolve my issue. Thanks for your help!