Trying to update an ArcGIS Online hosted feature serviceDescription using admin rest api

[ChrisCrowe]

I have an ArcGIS Online Hosted Feature Service.

I am trying to perform an updateDefinition and update the serviceDescription of a hostedFeatureService.

I am passing this as the payload to the rest service url

https://services.arcgis.com/abc/ArcGIS/rest/admin/services/CC_3_VMSData/FeatureServer/updateDefiniti...

This appears correct as per https://developers.arcgis.com/rest/services-reference/enterprise/update-definition-feature-service-....

But I get the following error

{
    "error": {
        "code": 400,
        "message": "Unable to update feature service definition.",
        "details": [
            "Invalid definition for 'ServiceDescription'.",
            "Exception has been thrown by the target of an invocation."
        ]
    }
}

I can only set the serviceDescription to "", anything else returns the above message. 

Any thoughts on how to overcome this?

Chris