diff --git a/src/scanpydoc/rtd_github_links/__init__.py b/src/scanpydoc/rtd_github_links/__init__.py
index fa36184..7f37675 100644
--- a/src/scanpydoc/rtd_github_links/__init__.py
+++ b/src/scanpydoc/rtd_github_links/__init__.py
@@ -171,7 +171,11 @@ def _get_linenos(obj: _SourceObjectType) -> tuple[int, int] | tuple[None, None]:
     """Get an object’s line numbers."""
     try:
         lines, start = inspect.getsourcelines(obj)
-    except TypeError:
+    # https://docs.python.org/3/library/inspect.html#inspect.getsourcelines
+    # means an OSError is raised if the source is not found,
+    # as is the case with collections.abc.Mapping.
+    # A TypeError indicates a builtin class.
+    except (TypeError, OSError):
         return None, None
     else:
         return start, start + len(lines) - 1
diff --git a/tests/test_rtd_github_links.py b/tests/test_rtd_github_links.py
index f2664e5..5289501 100644
--- a/tests/test_rtd_github_links.py
+++ b/tests/test_rtd_github_links.py
@@ -8,6 +8,7 @@
 from typing import TYPE_CHECKING
 from pathlib import Path, PurePosixPath
 from importlib import import_module
+from collections.abc import Mapping
 
 import pytest
 from sphinx.config import Config
@@ -149,6 +150,12 @@ def test_as_function(
     assert github_url(f"scanpydoc.{module}.{name}") == f"{prefix}/{obj_path}#L{s}-L{e}"
 
 
+@pytest.mark.parametrize("cls", [dict, Mapping])
+def test_no_line_nos_for_unavailable_source(cls: type) -> None:
+    start, end = _get_linenos(cls)
+    assert start is end is None
+
+
 def test_get_github_url_only_annotation(prefix: PurePosixPath) -> None:
     """Doesn’t really work but shouldn’t crash either."""
     url = github_url(f"{_testdata.__name__}.TestCls.test_anno")