I guess like many others, I place orders like this:
Order(ETF.Symbol, (int)(PortfolioValue / Symbol.Close ), OrderType.Market);
However, in reality, sometimes the price of a particular symbol is not available, and a "Divided by 0" error showed up. This will blow up my order process.
My question is how I can tell if the symbol price is unavailable at a certain time so I can kick it out and prevent this problem?

Thank you!