MTGox v2 api error " login_error_invalid_rest_sign" How to write sign?

[finbit]

Hi

When i trying send order via api i have problem(another  private api works fine like get orders ,get info

"result":"error","error":"Identification required to access private API","token":"login_error_invalid_rest_sign

What i should change?

Code:

 IMtGoxExchange mtGoxExchange = new MtGoxExchange();
                OrderCreateResponse response1 = mtGoxExchange.CreateOrder(Currency.USD, OrderType.Ask, 150.0, 10.0);
                OrderCancelResponse cancelResponse1 = mtGoxExchange.CancelOrder(response1.OID);
				
				inner source code for request (from debug window)
				prefix=BTCUSD/money/order/add
				post=nonce=635028379133977468&type=ask&amount_int=15000000000&price_int=1000000
				
				string sign = getHash(Convert.FromBase64String(APISecret), prefix + Convert.ToChar(0) + post);
				
				  req.AddHeader("Rest-Key", APIKey);
            req.AddHeader("Rest-Sign", sign);
            req.AddParameter("nonce", nonce);