Should in fact be

        first_div :: Integral a => a ; first_div = round (sqrt $ fromIntegral x)

Basically, sqrt has the type signature sqrt :: (Floating a) => a -> a, meaning it only works on things like doubles. The rest of your code only works on integral types. Your code compiles just fine because, in theory, you could have a data structure which in an instance of both Integral and Floating, but the argument you provide is probably just a regular old Int or something like that (I can't be certain, as you have not provided the code that calls isPrime1).

Let me know if you'd like further clarification.

Michael