Please read Please read: make it easier to help you, and try to produce an MWE (minimum working example) of your code, to make it easier for others to help you.
From what you posted it appears that your code is trying to convert the number 128 into an Int8 format, which is failing as Int8 only supports the range from -128 to 127 (see the docs here for more explanation of the different number types).
It is hard to say how you should modify your code without knowing what you are actually trying to do.