if-then-else expression works but cond does not in ocaml factorial function

let cond (a,b,c) = match a with | true -> t | false -> e

let rec fact n = cond (n=0,1, n * fact (n-1))

let rec fact n = if n=0 then 1 else n * fact (n-1)

in the above code segments, the first version gives a stack overflow exception while the second one works fine. what is the difference between these two? they seem to function the same but apparently don't.