diff --git a/Tests/test_pyroma.py b/Tests/test_pyroma.py
index 5871a7213..915dbe7b6 100644
--- a/Tests/test_pyroma.py
+++ b/Tests/test_pyroma.py
@@ -6,10 +6,15 @@ import pytest
 
 from PIL import __version__
 
+TYPE_CHECKING = False
+
+if TYPE_CHECKING:
+    from importlib.metadata import PackageMetadata
+
 pyroma = pytest.importorskip("pyroma", reason="Pyroma not installed")
 
 
-def map_metadata_keys(md):
+def map_metadata_keys(md: PackageMetadata) -> dict[str, str | list[str] | None]:
     # Convert installed wheel metadata into canonical Core Metadata 2.4 format.
     # This was a utility method in pyroma 4.3.3; it was removed in 5.0.
     # This implementation is constructed from the relevant logic from
@@ -17,16 +22,16 @@ def map_metadata_keys(md):
     # upstream to Pyroma as https://github.com/regebro/pyroma/pull/116,
     # so it may be possible to simplify this test in future.
     data = {}
-    for key in set(md.keys()):
+    for key in set(md):
         value = md.get_all(key)
         key = pyroma.projectdata.normalize(key)
 
-        if len(value) == 1:
-            value = value[0]
-            if value.strip() == "UNKNOWN":
-                continue
-
-        data[key] = value
+        if value is not None and len(value) == 1:
+            first_value = value[0]
+            if first_value.strip() != "UNKNOWN":
+                data[key] = first_value
+        else:
+            data[key] = value
     return data
 
 
diff --git a/pyproject.toml b/pyproject.toml
index f4514925d..cc616bc54 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -217,6 +217,7 @@ testpaths = [
 python_version = "3.10"
 pretty = true
 disallow_any_generics = true
+disallow_untyped_defs = true
 enable_error_code = "ignore-without-code"
 extra_checks = true
 follow_imports = "silent"