diff --git a/flit/common.py b/flit/common.py
index a3ae12d5..19b8d171 100644
--- a/flit/common.py
+++ b/flit/common.py
@@ -247,7 +247,11 @@ def __init__(self, data):
             assert hasattr(self, k), "data does not have attribute '{}'".format(k)
             setattr(self, k, v)
         if extras_require:
-            self.requires_dist = self.requires_dist + ['{}; extra == "{}"'.format(d, e) for e, d in extras_require.items()]
+            self.requires_dist = list(self.requires_dist) + [
+                '{}; extra == "{}"'.format(d, e)
+                for e, ds in extras_require.items()
+                for d in ds
+            ]
 
     def _normalise_name(self, n):
         return n.lower().replace('-', '_')
diff --git a/tests/samples/extras.toml b/tests/samples/extras.toml
index dbf296f8..c070986a 100644
--- a/tests/samples/extras.toml
+++ b/tests/samples/extras.toml
@@ -7,6 +7,7 @@ author = "Sir Robin"
 author-email = "robin@camelot.uk"
 home-page = "http://github.com/sirrobin/module1"
 description-file = "EG_README.rst"
+requires = ["toml"]
 
 [tool.flit.metadata.extras-require]
 test = ["pytest"]
diff --git a/tests/test_install.py b/tests/test_install.py
index df1c9e88..2d588d11 100644
--- a/tests/test_install.py
+++ b/tests/test_install.py
@@ -117,6 +117,36 @@ def test_install_requires(self):
         assert len(calls) == 1
         assert calls[0]['argv'][1:5] == ['-m', 'pip', 'install', '-r']
 
+@pytest.mark.parametrize(('deps', 'extras', 'installed'), [
+    ('none', [], set()),
+    ('dev', [], {'pytest;'}),  # TODO: why not also normal reqs, i.e. toml?
+    ('production', [], {'toml;'}),
+    ('all', [], {'toml;', 'pytest;', 'requests;'}),
+])
+def test_install_requires_extras(deps, extras, installed, samples_dir):
+    it = InstallTests()
+    try:
+        it.setUp()
+        ins = Installer(samples_dir / 'extras.toml', python='mock_python',
+                        user=False, deps=deps, extras=extras)
+
+        cmd = MockCommand('mock_python')
+        get_reqs = (
+            "#!{python}\n"
+            "import sys, pathlib\n"
+            "pathlib.Path({recording_file!r}).write_text(pathlib.Path(sys.argv[-1]).read_text())\n"
+        ).format(python=sys.executable, recording_file=cmd.recording_file)
+        cmd.content = get_reqs
+
+        with cmd as mock_py:
+            ins.install_requirements()
+            str_deps = pathlib.Path(mock_py.recording_file).read_text()
+            deps = str_deps.split('\n') if str_deps else []
+
+        assert set(deps) == installed
+    finally:
+        it.tearDown()
+
 def test_requires_dist_to_pip_requirement():
     rd = 'pathlib2 (>=2.3); python_version == "2.7"'
     assert _requires_dist_to_pip_requirement(rd) == \
diff --git a/tests/test_metadata.py b/tests/test_metadata.py
index 8cab3349..f24b6f89 100644
--- a/tests/test_metadata.py
+++ b/tests/test_metadata.py
@@ -11,4 +11,9 @@ def test_extras(samples_dir):
 def test_extras_dev_conflict(samples_dir):
     info = read_pkg_ini(samples_dir / 'extras-dev-conflict.toml')
     with pytest.raises(ValueError, match=r'Ambiguity'):
-        Metadata(dict(name=info['module'], version='0.0', summary='', **info['metadata']))
\ No newline at end of file
+        Metadata(dict(name=info['module'], version='0.0', summary='', **info['metadata']))
+
+def test_extra_conditions(samples_dir):
+    info = read_pkg_ini(samples_dir / 'extras.toml')
+    meta = Metadata(dict(name=info['module'], version='0.0', summary='', **info['metadata']))
+    assert set(meta.requires_dist) == {'toml', 'pytest; extra == "test"', 'requests; extra == "custom"'}