UpdateTabsAsync throws "INVALID_REQUEST_BODY: The request body is missing or improperly formatted. No tabs specified."

[PeterJamesLawson]

From EnvelopesApi.cs:

/// <summary>
/// Updates the tabs for a recipient.   Updates one or more tabs for a recipient in a draft envelope.
/// </summary>
/// <exception cref="DocuSign.eSign.Client.ApiException">Thrown when fails to make API call</exception>
/// <param name="accountId">The external account number (int) or account ID Guid.</param>
/// <param name="envelopeId">The envelopeId Guid of the envelope being accessed.</param>
/// <param name="recipientId">The ID of the recipient being accessed.</param>
/// <param name="tabs"> (optional)</param>
/// <returns>Task of Tabs</returns>
public async System.Threading.Tasks.Task<Tabs> UpdateTabsAsync(string accountId, string envelopeId, string recipientId, Tabs tabs = null)
{
        ApiResponse<Tabs> localVarResponse = await UpdateTabsAsyncWithHttpInfo(accountId, envelopeId, recipientId, tabs);
        return localVarResponse.Data;
}

However, either of these throw "INVALID_REQUEST_BODY: The request body is missing or improperly formatted. No tabs specified."

await EnvelopesApi.UpdateTabsAsync(accountId, envelopeId, "1");
await EnvelopesApi.UpdateTabsAsync(accountId, envelopeId, "1", null);

Any idea what I am doing wrong?