Problem when trying to find incoming texts with CallFire API

I am using the CallFire C# API to query for inbound text messages. When I execute the Find command, I am getting an InternalServerErrorException. I do not get this exception when querying for outbound text messages. Here is the minimal code necessary to reproduce the problem:

var client = new CallfireClient(m_username, m_password);
var request = new FindTextsRequest
{
    Inbound = true
};
Page<Text> texts = client.TextsApi.Find(request);

Here are the exception details:

CallfireApiClient.InternalServerErrorException
  HResult=0x80131500
  Source=callfire-api-client
  StackTrace:
   at CallfireApiClient.RestApiClient.VerifyResponse(IRestResponse response)
   at CallfireApiClient.RestApiClient.DoRequest[T](IRestRequest request)
   at CallfireApiClient.RestApiClient.Get[T](String path, IEnumerable`1 queryParams)
   at CallfireApiClient.RestApiClient.Get[T](String path, CallfireModel request)
   at CallfireApiClient.Api.CallsTexts.TextsApi.Find(FindTextsRequest request)

I have an inbound text message waiting--I can see it from the CallFire website. I need to be able to see it from the CallFire API.