Captureing of Bluetoothclient data with start byte value

My Bluetoothclient is receiving datas continously in every 1 sec ,it contains 12 byte s it is somewhat like " FDT 33000. " I want that numerical value as 33 so I use split at space and divide that value with 1000 .The problem is sometimes due to null or empty value run time error occurs. How to solve this issue?. If we recognize the data with first byte as "F" does it solve the problem? How can I implement?

Well, read the error message ..

What should be the result of that division?
Isn't it obvious, rhat this division does not make sense?

Upload your sketch, and also your exported .aia file, so we can compare them.

We need the .ino file too.

Your error message proves you wrong.
How many Fs in that?

That's why we need to see where this garbage comes from.