Why currying with obj doesn't work?

Why currying with obj doesn't work?

Can you explain why one works but not the other?
Given
//fu : unit -> unit
let fu() = ();;
This works
//exec : (unit -> unit) -> int -> unit
let exec (f:(unit -> unit)) (data:int) = f();;
//this works, and p : int -> unit
let p = exec fu;;
And it works for other types of data such as string, long, etc.
This doesn't work
//exec : (unit -> unit) -> obj -> unit
let exec (f:(unit -> unit)) (data:obj) = f();;
let p = exec fu;;
and I get the following error:
error FS0030: Value restriction. The value 'p' has been inferred to have
generic type val p : ('_a -> unit)
Either make the arguments to 'p' explicit or, if you do not intend for it
to be generic, add a type annotation.
Notice the only difference between these cases is the type of the data
parameter. When it is obj or System.Object or 'a - it doesn't work.
Another thing is that if data has type obj then the following happens:
//Data type is obj
let exec (f:(unit -> unit)) (data:obj) = f();;
//specifying parameters explicitly
let p x = exec fu x;;
Now p has the signature of 'a -> unit, not obj -> unit.
So the question is: why the "shortcuted" currying doesn't work when data
is obj or 'a and why the type of p is 'a -> unit when data was obj?