ResourceRegistrationFailureException when calling the RegisterThing operation - thing groups conflict - amazon-web-services

Question

I am getting the following error when trying to use the RegisterThings action from AWS IoT. Within the API, call, I am trying to set the thing groups but getting the following error:
An error occurred (ResourceRegistrationFailureException) when calling the RegisterThing operation: Existing ThingGroups: [] conflict with ThingGroups: [mythingggroup] in Template
Here is my provisioning template:
provision_json = json.dumps({
"Parameters": {
"ThingName": {
"Type": "String"
},
"ThingGroup": {
"Type": "String"
},
"CertificateId": {
"Type": "String"
},
"PolicyName": {
"Type": "String"
},
"AssumeRolePolicyName": {
"Type": "String"
}
},
"Resources": {
"Thing": {
"Type": "AWS::IoT::Thing",
"Properties": {
"ThingName": {
"Ref": "ThingName"
},
"ThingGroups": [{
"Ref": "ThingGroup"
}]
},
"OverrideSettings" : {
"ThingGroups" : "REPLACE"
}
},
"Certificate": {
"Type": "AWS::IoT::Certificate",
"Properties": {
"CertificateId": {
"Ref": "CertificateId"
}
}
},
"DevicePolicy": {
"Type": "AWS::IoT::Policy",
"Properties": {
"PolicyName": {
"Ref": "PolicyName"
}
}
},
"TESAssumeRolePolicy": {
"Type": "AWS::IoT::Policy",
"Properties": {
"PolicyName": {
"Ref": "AssumeRolePolicyName"
}
}
}
}
})
then I call the boto3 client like this:
iot = boto3.client("iot")
response = iot.register_thing(
templateBody=provision_json,
parameters={
"CertificateId": "1234",
"ThingName": "mythingname",
"ThingGroup": "mythinggroupname",
"PolicyName": "mypolicyname",
"AssumeRolePolicyName": "anotherpolicyname"
}
)
I thought this might be a permissions error, but even after I give the caller iot:*, I still have the same issue.
AWS isn't providing great documentation on why this error is occurring. Does anyone have any ideas?