Syntax for warning options with ocamlc (and dune)

I want my dune utop lib command to compile with warnings number 27 and 29 disabled, and number 28 enabled. So my dune file is as follows :

(include_subdirs unqualified)
(flags w-27+28-29)
(name skeptical_duck_lib))

I get the following error message :

$ dune utop lib
File “lib/.skeptical_duck_lib.objs/byte/unknown”, line 1, characters 0-0:
don’t know what to do with w-27+28-29

What would be the correct syntax then?

try a -w -27+28-29