Haskell ambiguous type

Question

findMult lst n = [x | x <- lst, x `mod` n == 0]

primes num = 
    let n = [2..num]
        x = ceiling (sqrt num)
        nsqrt = [2..x]
        not_prime = map (findMult n) nsqrt
    in diff2 n (concat not_prime)   

has the following problem when i try to run it

<interactive>:1:0:
    Ambiguous type variable `t' in the constraints:
      `RealFrac t' arising from a use of `primes' at <interactive>:1:0-8
      `Floating t' arising from a use of `primes' at <interactive>:1:0-8
      `Integral t' arising from a use of `primes' at <interactive>:1:0-8
    Probable fix: add a type signature that fixes these type variable(s)

I tried using fromIntegral but i don't think i used correctly as that gives me compilation error. Please help.

The purpose of this is to find all the prime numbers up until num.