API - Update Test Cycle - Invalid JSON Error 151

I'm using the API to update Zephyr Squad within JIRA. Whether through my scripting, or directly through https://zephyrsquad.docs.apiary.io/ (even using the format of the example body), I'm receiving an Invalid JSON error when updating an existing test cycle -

URL: https://prod-api.zephyr4jiracloud.com/connect/public/rest/api/1.0/cycle/[CYCLE_ID]

Headers: Content-Type (application/json), Authorization (script generated JWT), and zapiAccessKey

Body:

{ "name": "10.11.1.3.1", "build": "10.11.1.3.1", "environment": "auto", "description": "DESCRIPTION", "startDate": "2022-03-02", "endDate": "2022-03-03", "versionId": [VERSION_ID], "projectId": [PROJECT_ID], "id": "[CYCLE_ID]" }

Response:

• content-type:application/json;charset=utf-8
• content-length:68
• access-control-allow-origin:*
• access-control-allow-methods:OPTIONS,GET,HEAD,POST,PUT,DELETE,TRACE,CONNECT
• access-control-expose-headers:
• access-control-max-age:10
• date:Wed, 09 Mar 2022 20:35:09 GMT
• connection:close
• {
• "errorType": "ERROR",
• "clientMessage": "Invalid Json",
• "errorCode": 151
• }

Also tried with the apiary example's "clearCustomFieldsFlag": true and "sprintId": null.

Any obvious fields that I'm missing, or formatting issues with the JSON? 

Many Thanks.