'system.argumentexception' When uploading multipart

I am trying to post multipart data using System.Net.Http.HttpClient but when I am instanciating my content I am getting this exception:

A first chance exception of type 'System.ArgumentException' occurred in System.Net.Http.DLL
An exception of type 'System.ArgumentException' occurred in System.Net.Http.DLL and wasn't handled before a managed/native boundary
e: System.ArgumentException: The format of value '---###---' is invalid.
Parameter name: boundary
   at System.Net.Http.MultipartContent.ValidateBoundary(String boundary)
   at System.Net.Http.MultipartContent..ctor(String subtype, String boundary)
   at System.Net.Http.MultipartFormDataContent..ctor(String boundary)
   at RestClientUploadFoto.MultipartStackOverflow.<postMultipart>d__2.MoveNext()

• Why does it happen?

here is the method in where I am getting the exception:

public async Task postMultipart()
{
    var client = new HttpClient();
    client.DefaultRequestHeaders.TryAddWithoutValidation("Content-Type", "multipart/form-data");

    // boundary      
    String boundary = "---###---"; // should never occur in your data

    // This is the postdata
    MultipartFormDataContent content = new MultipartFormDataContent(boundary);
    content.Add(new StringContent("12", Encoding.UTF8), "userId");
    content.Add(new StringContent("78", Encoding.UTF8), "noOfAttendees");
    content.Add(new StringContent("chennai", Encoding.UTF8), "locationName");
    content.Add(new StringContent("32.56", Encoding.UTF8), "longitude");
    content.Add(new StringContent("32.56", Encoding.UTF8), "latitude");

    Console.Write(content);
    // upload the file sending the form info and ensure a result.
    // it will throw an exception if the service doesn't return a valid successful status code
    await client.PostAsync(fileUploadUrl, content)
        .ContinueWith((postTask) =>
        {
            postTask.Result.EnsureSuccessStatusCode();
        });

}