"Root element is missing." XML Error when trying to add Requirement through .NET API

I am trying to add a Requirement through the .NET API. Here is my code:

//creating new connection

HttpServerConnection conn = IomFactory.CreateHttpServerConnection(serverUrl, dbName, username, password);
Innovator inn = IomFactory.CreateInnovator(conn);
conn.Login();

Item addItem = inn.newItem("re_Requirement", "add");
Dictionary<string, string> fieldDict = new Dictionary<string, string>();

//adding properties
fieldDict.Add("managed_by_id", "AD30A6D8D3B642F5A2AFED1A4B02BEFA");
fieldDict.Add("req_priority", "Critical");
fieldDict.Add("classification", "Test");
fieldDict.Add("owned_by_id", "AD30A6D8D3B642F5A2AFED1A4B02BEFA");
fieldDict.Add("req_risk", "Low");
fieldDict.Add("type", "Text");

foreach (KeyValuePair<string,string> field in fieldDict)
{
    addItem.setProperty(field.Key, field.Value);
}

Item result = addItem.apply();
Console.WriteLine(result);
Console.WriteLine(result.isError()); // returns true

When I run this program in Visual Studio, I get the following error:

<SOAP-ENV:Envelope xmlns:SOAP-ENV="">schemas.xmlsoap.org/.../"><SOAP-ENV:Body><SOAP-ENV:Fault xmlns:af="">www.aras.com/.../faultcode><faultstring><![CDATA[Root element is missing.]]></faultstring><detail><af:legacy_detail><![CDATA[Root element is missing.]]></af:legacy_detail><af:exception message="Root element is missing." type="System.Xml.XmlException" /></detail></SOAP-ENV:Fault></SOAP-ENV:Body></SOAP-ENV:Envelope>

I am using Aras 11 SP15. Note that this has the Requirements Engineering feature only instead of the Requirements Management, hence the name of the item is re_Requirement. How to solve this issue?