illegal guard expression: filelib:is_dir(H)

[Kay Kay]

I have a function along the following lines

do_process_elements([],  _) ->   [];
do_process_elements([H | _], Fn) ->
   if
     filelib:is_dir(H) ->
       process_directory(H, Fn);
     true ->
       process_file(H, Fn)
   end.


When I compile - I get the following error,  where the line number 
refers to    - filelib:is_dir(H) function.

./test.erl:23: illegal guard expression

Given that filelib:is_dir(H) returns true / false  - I thought this 
would fit in here as a predicate. What could be going wrong ?