AWS Athena struct not parsing JSON string

I am using AWS Athena to do some queries on AWS CloudTrail data object log entries.

The first few fields in a typical log entry look like this (pretty-printed for clarity):

{
  "Records": [
    {
      "eventVersion": "1.08",
      "userIdentity": {
        "type": "AWSAccount",
        "principalId": "",
        "accountId": "ANONYMOUS_PRINCIPAL"
      },
      "eventTime": "2021-03-23T14:04:38Z",
      "eventSource": "s3.amazonaws.com",
      "eventName": "GetObject",
      "awsRegion": "us-east-1",
      "sourceIPAddress": "12.34.45.56",
      "userAgent": "[Amazon CloudFront]",
      "requestParameters": {
        "bucketName": "mybucket",
        "Host": "mybucket.s3.amazonaws.com",
        "key": "bin/some/path/to/a/file"
      },
      "responseElements": null,
...

The AWS CloudTrail console will create a standard table to query these entries. The table is defined as this:

CREATE EXTERNAL TABLE `cloudtrail_logs_mybucket_logs`(
  `eventversion` string COMMENT 'from deserializer', 
  `useridentity` struct<type:string,principalid:string,arn:string,accountid:string,invokedby:string,accesskeyid:string,username:string,sessioncontext:struct<attributes:struct<mfaauthenticated:string,creationdate:string>,sessionissuer:struct<type:string,principalid:string,arn:string,accountid:string,username:string>>> COMMENT 'from deserializer', 
  `eventtime` string COMMENT 'from deserializer', 
  `eventsource` string COMMENT 'from deserializer', 
  `eventname` string COMMENT 'from deserializer', 
  `awsregion` string COMMENT 'from deserializer', 
  `sourceipaddress` string COMMENT 'from deserializer', 
  `useragent` string COMMENT 'from deserializer', 
  `errorcode` string COMMENT 'from deserializer', 
  `errormessage` string COMMENT 'from deserializer', 
  `requestparameters` string COMMENT 'from deserializer', 
  `responseelements` string COMMENT 'from deserializer', 
  `additionaleventdata` string COMMENT 'from deserializer', 
  `requestid` string COMMENT 'from deserializer', 
  `eventid` string COMMENT 'from deserializer', 
  `resources` array<struct<arn:string,accountid:string,type:string>> COMMENT 'from deserializer', 
  `eventtype` string COMMENT 'from deserializer', 
  `apiversion` string COMMENT 'from deserializer', 
  `readonly` string COMMENT 'from deserializer', 
  `recipientaccountid` string COMMENT 'from deserializer', 
  `serviceeventdetails` string COMMENT 'from deserializer', 
  `sharedeventid` string COMMENT 'from deserializer', 
  `vpcendpointid` string COMMENT 'from deserializer')
COMMENT 'CloudTrail table for adafruit-circuit-python-logs bucket'
ROW FORMAT SERDE 
  'com.amazon.emr.hive.serde.CloudTrailSerde' 
STORED AS INPUTFORMAT 
  'com.amazon.emr.cloudtrail.CloudTrailInputFormat' 
OUTPUTFORMAT 
  'org.apache.hadoop.hive.ql.io.HiveIgnoreKeyTextOutputFormat'
LOCATION
  's3://mybucket/AWSLogs/12345678901234/CloudTrail'
TBLPROPERTIES (
  'classification'='cloudtrail', 
  'transient_lastDdlTime'='1616514617')

Note that useridentity is described as a struct, but requestParameters is a string. I would like to use the struct feature to preparse requestParameters, so I tried this:

CREATE EXTERNAL TABLE `cloudtrail_logs_mybucket_logs2`(
  `eventversion` string COMMENT 'from deserializer', 
  `useridentity` struct<type:string,principalid:string,arn:string,accountid:string,invokedby:string,accesskeyid:string,username:string,sessioncontext:struct<attributes:struct<mfaauthenticated:string,creationdate:string>,sessionissuer:struct<type:string,principalid:string,arn:string,accountid:string,username:string>>> COMMENT 'from deserializer', 
  `eventtime` string COMMENT 'from deserializer', 
  `eventsource` string COMMENT 'from deserializer', 
  `eventname` string COMMENT 'from deserializer', 
  `awsregion` string COMMENT 'from deserializer', 
  `sourceipaddress` string COMMENT 'from deserializer', 
  `useragent` string COMMENT 'from deserializer', 
  `errorcode` string COMMENT 'from deserializer', 
  `errormessage` string COMMENT 'from deserializer', 
  `requestparameters` struct<`bucketName`:string, `Host`:string, `key`:string> COMMENT 'THIS IS NEW', 
...[rest same as above]

The table is created, but trying to do a simple query using it ("Preview Table") gives this error:

GENERIC_INTERNAL_ERROR: parent builder is null

What's wrong with my attempt to use struct on requestparameters? It seems no different in terms of JSON as to what's going on with useridentity.