--fixtures only shows fixtures from first file

[The-Compiler]

If I have two files:

test_a.py:

import pytest

@pytest.fixture
def fix_a():
    """Fixture A"""
    pass

def test_a(fix_a):
    pass

and test_b.py:

import pytest

@pytest.fixture
def fix_b():
    """Fixture B"""
    pass

def test_b(fix_b):
    pass

Running pytest --fixtures will only show fix_a:

====================== test session starts ======================
platform linux -- Python 3.4.3 -- py-1.4.30 -- pytest-2.7.2
rootdir: /home/florian/tmp/fixture_test, inifile: 
plugins: mock
collected 2 items 
[...]

----------------- fixtures defined from test_a ------------------
fix_a
    Fixture A

=======================  in 0.01 seconds ========================