diff --git a/flit/common.py b/flit/common.py
index 13a888bd..a3ae12d5 100644
--- a/flit/common.py
+++ b/flit/common.py
@@ -229,9 +229,6 @@ class Metadata:
 
     metadata_version = "2.1"
 
-    # deprecated, we use “extras_require: {'dev': ...}” for it
-    dev_requires = ()
-
     def __init__(self, data):
         self.name = data.pop('name')
         self.version = data.pop('version')
@@ -240,7 +237,9 @@ def __init__(self, data):
         extras_require = data.pop('extras_require', {})
         dev_requires = data.pop('dev_requires', None)
         if dev_requires is not None:
-            log.warning('“dev_requires = ...” is obsolete. Use “extras_require = {"dev" = ...}” instead.')
+            if 'dev' in extras_require:
+                raise ValueError('Ambiguity: Encountered dev-requires together with its replacement extras-require.dev.')
+            log.warning('“dev-requires = ...” is obsolete. Use “extras-require = {"dev" = ...}” instead.')
             extras_require.setdefault('dev', []).extend(dev_requires)
         explicit_extras = data.pop('provides_extra', ())
         self.provides_extra = list(set(explicit_extras) | extras_require.keys())
diff --git a/tests/samples/dev_requires_with_empty_lines.ini b/tests/samples/dev_requires_with_empty_lines.ini
deleted file mode 100644
index 31653d24..00000000
--- a/tests/samples/dev_requires_with_empty_lines.ini
+++ /dev/null
@@ -1,7 +0,0 @@
-[metadata]
-module=module1
-author=Sir Robin
-author-email=robin@camelot.uk
-home-page=http://github.com/sirrobin/module1
-dev-requires=
-	foo
diff --git a/tests/test_inifile.py b/tests/test_inifile.py
index c7d3e5fb..0c5d9521 100644
--- a/tests/test_inifile.py
+++ b/tests/test_inifile.py
@@ -18,13 +18,9 @@ def test_classifiers_with_space():
     """
     read_pkg_ini(samples_dir / 'classifiers_with_space.ini')
 
-@pytest.mark.parametrize(('filename', 'key', 'expected'), [
-    ('requires_with_empty_lines.ini', 'requires_dist', ['foo', 'bar']),
-    ('dev_requires_with_empty_lines.ini', 'dev_requires', ['foo']),
-])
-def test_requires_with_empty_lines(filename, key, expected):
-    ini_info = read_pkg_ini(samples_dir / filename)
-    assert ini_info['metadata'][key] == expected
+def test_requires_with_empty_lines():
+    ini_info = read_pkg_ini(samples_dir / 'requires_with_empty_lines.ini')
+    assert ini_info['metadata']['requires_dist'] == ['foo', 'bar']
 
 def test_missing_entrypoints():
     with pytest.raises(FileNotFoundError):