Why does this have return code 0 when executed?

Makefile

default:
	ocamlfind ocamlc -o test_test -package oUnit -linkpkg -g test.ml

clean:
	rm *.cmi *.cmo *.cmx test_test

test.ml

open OUnit

let test1 () = assert_failure "Test failure"

let suite = "suite" >::: ["test1" >:: test1]

let _ =
	run_test_tt_main suite

For me, running the executable from the above ./test_test states there was a failure but the executable returns 0. Contrary to the following docs for OUnit:

When using OUnit.run_test_tt_main, a non zero exit code signals that the test suite was not successful.