Absolute value for floats in core OCaml

I am in need of an absolute value function for floats in OCaml and the core language doesn't seem to possess one, so I wrote the following:

let absF (f:float) = if f > 0.0 then f else (f *. -1.0);;

which seems to work for positives but not for negatives, citing:

This expression has type float -> float but is here used with type int

What is the error in my logic?