Problem With insert Query With Where Conditions

Question

Am Using Query Like this

insert into BalanceSheet (Balance) select @Balance FROM OrderBill WHERE CustomerID=@CustomerID and InvoiceID=@InvoiceID

Getting Error Like

Cannot insert the value NULL into column 'CustID', table 'BillingAndAccounts.dbo.BalanceSheet'; column does not allow nulls. INSERT fails.

The statement has been terminated.