Invoking Lambda function from another Lambda function consistently results in a strange HTTP 421 Misdirected Request error

I'm suffering from a bizarre undocumented HTTP 421 Misdirected Request error when trying to invoke Lambda functions from other Lambda functions via Python/boto3 (with InvocationType = 'Event') like so:

lambda_client.invoke(
    FunctionName   = my_lambda_function,
    Payload        = json.dumps('{'foo': 'bar'}),
    InvocationType = 'Event'
)

The error thrown is:

botocore.exceptions.ClientError: An error occurred (421) when calling the Invoke operation:

(With nothing printed beyond the colon.)

I believe my Lambda permission are in order too, as per below?

Execution role:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Principal": {
        "Service": "lambda.amazonaws.com"
      },
      "Action": "sts:AssumeRole"
    }
  ]
}

Execution role policy (inline):

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "lambda:InvokeFunction"
      ],
      "Resource": [
        "arn:aws:lambda:MY_AWS_REGION:MY_AWS_ACCOUNT_ID:function:MY_LAMBDA_FUNCTION_NAME"
      ]
    }
  ]
}

Anyone have any idea?

https://docs.aws.amazon.com/lambda/latest/api/API_Invoke.html lists a bunch of possible error codes, but they do not include any mention of HTTP 421 or Misdirected Request. Worse, Googling "421" Lambda invoke does not seem to bring up anything relevant at all, either.

I was able to repro the exact same error ClientError: An error occurred (421) when calling the Invoke operation by passing \n to FunctionName:

client.invoke(
  FunctionName='MyOtherFunction\n',
  InvocationType = 'Event'
)

MyOtherFunction is the name of the function I want to invoke, and \n is what I added to repro the error.

So, please make sure you are not passing weird characters to FunctionName such as \n or %0a (new line symbol).