diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 33b64d7..c89a46b 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -29,12 +29,9 @@ jobs:
       - name: Install
         run: |
           python -m pip install --upgrade pip
-          make setup
           pip install -U .
       - name: Test
-        run: make test
-      - name: Lint
-        run: make lint
+        run: python -m metadata_please.tests
 
   metadata_please:
     runs-on: ${{ matrix.os }}
diff --git a/metadata_please/sdist.py b/metadata_please/sdist.py
index c2bd5c2..743d673 100644
--- a/metadata_please/sdist.py
+++ b/metadata_please/sdist.py
@@ -12,12 +12,15 @@ def from_zip_sdist(zf: ZipFile) -> bytes:
     """
     requires = [f for f in zf.namelist() if f.endswith("/requires.txt")]
     requires.sort(key=len)
+    if not requires:
+        return b""
+
     data = zf.read(requires[0])
     assert data is not None
-    requires, extras = convert_sdist_requires(data.decode("utf-8"))
+    requires_lines, extras = convert_sdist_requires(data.decode("utf-8"))
 
     buf: list[str] = []
-    for req in requires:
+    for req in requires_lines:
         buf.append(f"Requires-Dist: {req}\n")
     for extra in sorted(extras):
         buf.append(f"Provides-Extra: {extra}\n")
@@ -27,6 +30,9 @@ def from_zip_sdist(zf: ZipFile) -> bytes:
 def basic_metadata_from_zip_sdist(zf: ZipFile) -> BasicMetadata:
     requires = [f for f in zf.namelist() if f.endswith("/requires.txt")]
     requires.sort(key=len)
+    if not requires:
+        return BasicMetadata((), frozenset())
+
     data = zf.read(requires[0])
     assert data is not None
     return BasicMetadata.from_sdist_pkg_info_and_requires(b"", data)
@@ -39,14 +45,16 @@ def from_tar_sdist(tf: TarFile) -> bytes:
     # XXX Why do ZipFile and TarFile not have a common interface ?!
     requires = [f for f in tf.getnames() if f.endswith("/requires.txt")]
     requires.sort(key=len)
+    if not requires:
+        return b""
 
     fo = tf.extractfile(requires[0])
     assert fo is not None
 
-    requires, extras = convert_sdist_requires(fo.read().decode("utf-8"))
+    requires_lines, extras = convert_sdist_requires(fo.read().decode("utf-8"))
 
     buf: list[str] = []
-    for req in requires:
+    for req in requires_lines:
         buf.append(f"Requires-Dist: {req}\n")
     for extra in sorted(extras):
         buf.append(f"Provides-Extra: {extra}\n")
@@ -57,6 +65,8 @@ def basic_metadata_from_tar_sdist(tf: TarFile) -> BasicMetadata:
     # XXX Why do ZipFile and TarFile not have a common interface ?!
     requires = [f for f in tf.getnames() if f.endswith("/requires.txt")]
     requires.sort(key=len)
+    if not requires:
+        return BasicMetadata((), frozenset())
 
     fo = tf.extractfile(requires[0])
     assert fo is not None
diff --git a/metadata_please/tests/sdist.py b/metadata_please/tests/sdist.py
index e0ecd93..529b29e 100644
--- a/metadata_please/tests/sdist.py
+++ b/metadata_please/tests/sdist.py
@@ -41,11 +41,23 @@ def test_basic_metadata(self) -> None:
         )
         bm = basic_metadata_from_zip_sdist(z)  # type: ignore
         self.assertEqual(
-            ["a", "b; extra == 'e'"],
+            ("a", "b; extra == 'e'"),
             bm.reqs,
         )
         self.assertEqual({"e"}, bm.provides_extra)
 
+    def test_basic_metadata_no_requires_file(self) -> None:
+        z = MemoryZipFile(
+            ["foo.egg-info/PKG-INFO", "foo/__init__.py"],
+            read_value=b"\n",
+        )
+        bm = basic_metadata_from_zip_sdist(z)  # type: ignore
+        self.assertEqual(
+            (),
+            bm.reqs,
+        )
+        self.assertEqual(set(), bm.provides_extra)
+
     def test_basic_metadata_absl_py_09(self) -> None:
         z = MemoryZipFile(
             ["foo.egg-info/requires.txt", "foo/__init__.py"],
@@ -60,12 +72,12 @@ def test_basic_metadata_absl_py_09(self) -> None:
         )
         bm = basic_metadata_from_zip_sdist(z)  # type: ignore
         self.assertEqual(
-            [
+            (
                 "six",
                 'enum34; python_version < "3.4"',
                 # Quoting on the following line is an implementation detail
                 "pytest; (python_version < \"3.4\") and extra == 'test'",
-            ],
+            ),
             bm.reqs,
         )
         self.assertEqual({"test"}, bm.provides_extra)
@@ -102,7 +114,7 @@ def test_basic_metadata(self) -> None:
         )
         bm = basic_metadata_from_tar_sdist(t)  # type: ignore
         self.assertEqual(
-            ["a", "b; extra == 'e'"],
+            ("a", "b; extra == 'e'"),
             bm.reqs,
         )
         self.assertEqual({"e"}, bm.provides_extra)
diff --git a/metadata_please/types.py b/metadata_please/types.py
index 12a0fcf..368f83e 100644
--- a/metadata_please/types.py
+++ b/metadata_please/types.py
@@ -11,14 +11,14 @@ class BasicMetadata:
     # Popualted from Requires-Dist or requires.txt
     reqs: Sequence[str]
     # Populated from Provides-Extra
-    provides_extra: set[str]
+    provides_extra: frozenset[str]
 
     @classmethod
     def from_metadata(cls, metadata: bytes) -> BasicMetadata:
         msg = message_from_string(metadata.decode("utf-8"))
         return BasicMetadata(
             msg.get_all("Requires-Dist") or (),
-            set(msg.get_all("Provides-Extra") or ()),
+            frozenset(msg.get_all("Provides-Extra") or ()),
         )
 
     @classmethod
@@ -33,7 +33,7 @@ def from_sdist_pkg_info_and_requires(
         )
 
 
-def convert_sdist_requires(data: str) -> tuple[list[str], set[str]]:
+def convert_sdist_requires(data: str) -> tuple[tuple[str, ...], frozenset[str]]:
     # This is reverse engineered from looking at a couple examples, but there
     # does not appear to be a formal spec.  Mentioned at
     # https://setuptools.readthedocs.io/en/latest/formats.html#requires-txt
@@ -64,4 +64,4 @@ def convert_sdist_requires(data: str) -> tuple[list[str], set[str]]:
                 lst.append(f"{line}; {current_markers}")
             else:
                 lst.append(line)
-    return lst, extras
+    return tuple(lst), frozenset(extras)