Specified Cast Is Not Valid

im doing a sum on a table and it either returns a number in decimal format or 'null' .  The problem is when it returns null i want it to just make the text say '0.00'.  So i did a test on the object that if it returns NULL just print  '0.00' but if it is not null it tells me that there is a number there and i want to store that as a decimal and print it out.  But i get an error for a type cast when im not it should not even be going to that part of the code.
 In the code below the first executescaler will return null so it should just go straight to the else.  But it gives me the type cast error in the if that shouldnt be seen.  The error and code are below. //Borrower NSF FEES
cmd.CommandText = "select sum(itemamount) from postmtdtls where loanid='" + LoanID + "' and Transactioncode = '310'";
object temp = cmd.ExecuteScalar();
if (temp != null)
{
decimal B_NSFFees = ((decimal)cmd.ExecuteScalar());
borrowerPayoff_NSFFees.Text = String.Format("{0:#,#.##}", B_NSFFees).ToString(); //Borrower NSF FEES
}
else
{
borrowerPayoff_NSFFees.Text = "0.00"; //borrowerPayoff_NSFFees.Text = "0.00";
} 
Server Error in '/WebSite5' Application. Specified cast is not valid. Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.InvalidCastException: Specified cast is not valid.

Source Error:

Line 774: if (temp != null)
Line 775: {
Line 776: decimal B_NSFFees = ((decimal)cmd.ExecuteScalar());
Line 777: borrowerPayoff_NSFFees.Text = String.Format("{0:#,#.##}", B_NSFFees).ToString(); //Borrower NSF FEES
Line 778: }
Source File: c:ProgrammingFilesWebSite5InvestorPool.aspx.cs    Line: 776

Stack Trace:

[InvalidCastException: Specified cast is not valid.]
InvestorPool.GetLoanInfo(String LoanID) in c:ProgrammingFilesWebSite5InvestorPool.aspx.cs:776
InvestorPool.MortAccountText(Object sender, EventArgs e) in c:ProgrammingFilesWebSite5InvestorPool.aspx.cs:660
System.Web.UI.WebControls.TextBox.OnTextChanged(EventArgs e) +75
System.Web.UI.WebControls.TextBox.RaisePostDataChangedEvent() +124
System.Web.UI.WebControls.TextBox.System.Web.UI.IPostBackDataHandler.RaisePostDataChangedEvent() +7
System.Web.UI.Page.RaiseChangedEvents() +138
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +4507

Version Information: Microsoft .NET Framework Version:2.0.50727.42; ASP.NET Version:2.0.50727.42