diff --git a/app/main.py b/app/main.py
index d478a2c..9c123da 100644
--- a/app/main.py
+++ b/app/main.py
@@ -162,7 +162,7 @@ async def _fetch_source_username(
 ) -> tuple[str, tuple[dict[str, int], bool] | Exception]:
     try:
         return username, await _fetch_with_cache(cache, key, fetcher)
-    except Exception as exc:
+    except (GitHubSourceError, GiteaSourceError) as exc:
         return username, exc
 
 
diff --git a/tests/test_main.py b/tests/test_main.py
index c25f8f3..f71e0fb 100644
--- a/tests/test_main.py
+++ b/tests/test_main.py
@@ -65,6 +65,25 @@ async def test_fetch_source_usernames_skips_failed_accounts() -> None:
     assert stale is False
 
 
+@pytest.mark.asyncio
+async def test_fetch_source_usernames_does_not_swallow_programming_errors() -> None:
+    def broken_fetcher(username):
+        async def fetch():
+            raise TypeError("bad shape")
+
+        return fetch
+
+    with pytest.raises(TypeError, match="bad shape"):
+        await _fetch_source_usernames(
+            None,
+            "github",
+            ["octocat", "hubot"],
+            date(2026, 1, 1),
+            date(2026, 1, 1),
+            broken_fetcher,
+        )
+
+
 @pytest.mark.asyncio
 async def test_fetch_source_usernames_keeps_cache_after_username_list_changes() -> None:
     with tempfile.TemporaryDirectory(dir=Path.cwd()) as temp_dir: