Can't create a signature request using .net SDK

I’m trying to use the .NET SDK to create a signature request for a file. My code is:

public async Task<BoxSignRequest> CreateSignatureRequest(BoxUser user, string signerEmail, string emailMessage, string subject, string sourceFileId, string destinationFolderId)
{
    var userClient = GetUserClient(user); //this uses the Client Credentials Grant to get a token for the user that owns the file
    BoxSignRequestCreateRequest createRequest = new BoxSignRequestCreateRequest()
    {
        AreRemindersEnabled = true,
        AreTextSignaturesEnabled = false,
        DaysValid = 10,
        EmailMessage = emailMessage,
        EmailSubject = subject,
        ParentFolder = new BoxRequestEntity()
        {
            Id = destinationFolderId
        },
        Signers = new List<BoxSignRequestSignerCreate>() {
            new BoxSignRequestSignerCreate()
            {
                Email = signerEmail,
                LoginRequired = true,
                Role = BoxSignRequestSignerRole.signer
            }
        },
        SourceFiles = new List<BoxSignRequestCreateSourceFile>()
        {
            new BoxSignRequestCreateSourceFile()
            {
                Id= sourceFileId
            }
        }
    };

    var signRequest =  await userClient.SignRequestsManager.CreateSignRequestAsync(createRequest);
    return signRequest;
}

I’m getting a Bad Request error. The documentation on the .NET SDK is not very comprehensive so I’m wondering if anyone else can help me out here.
Thanks!