Bug to Report in the boto3 sdk for ELBv2 for the register_targets call

I'd like to report a bug with the python boto3 sdk for ELBv2, specifically the register_targets call.

When I try to make a call using the boto3 sdk for ELBv2 to make the register_targets call, I get the following error:

Error Registering Targets: InternalFailure: {'error': 'An error occurred (InternalFailure) when calling the RegisterTargets operation (reached max retries: 4): Unknown'}

However, I know that the call is correct because when use the exact same call with the aws cli it works perfectly and creates the target groups in question.

NOTE: I'm registering ip targets pointing at ENIs to the target group.

boto3 call details (with ids obfuscated):

target_group_arn = "arn:aws:elasticloadbalancing:us-east-1:ACCOUNTID:targetgroup/TARGETGROUPNAME/TARGETGROUPID"

client = boto3.client('elbv2')

response = client.register_targets( TargetGroupArn=target_group_arn, Targets=[ { "Id": "10.0.X.XYZ", "Port": 443, "AvailabilityZone": "us-east-1a" }, { "Id": "10.0.XYZ.XYZ", "Port": 443, "AvailabilityZone": "us-east-1b" } ] )

AWS CLI call details

aws elbv2 register-targets --cli-input-json "{"TargetGroupArn":"arn:aws:elasticloadbalancing:us-east-1:ACCOUNTID:targetgroup/TARGETGROUPNAME/TARGETGROUPID","Targets":[{"Id":"10.0.X.XYZ","Port":443,"AvailabilityZone":"us-east-1a"},{"Id":"10.0.XYZ.XYZ","Port":443,"AvailabilityZone":"us-east-1b"}]}"

But the CLI call works and the boto3 call does not.

I've tried tens of possible combinations trying to figure this out. I've tried different formats for the boto3 calls but I get formatting errors, so I know at least the formatting checking is working on the register_targets boto3 call.

Please help me AWS.