diff --git a/requirements/dev-mypy.txt b/requirements/dev-mypy.txt
index 4fc6daeb..b2a8f056 100644
--- a/requirements/dev-mypy.txt
+++ b/requirements/dev-mypy.txt
@@ -10,3 +10,4 @@ pytest
 pydantic>=2.0
 pydantic-extra-types
 sphinx>=4.5
+-r black.txt
diff --git a/sphinx_immaterial/apidoc/python/object_ids.py b/sphinx_immaterial/apidoc/python/object_ids.py
index e6435374..0f95c788 100644
--- a/sphinx_immaterial/apidoc/python/object_ids.py
+++ b/sphinx_immaterial/apidoc/python/object_ids.py
@@ -98,8 +98,15 @@ def strip_object_entry_node_id(existing_node_id: str, object_id: str):
         noindexentry = "noindexentry" in self.options
 
         for signode in cast(List[docutils.nodes.Element], signodes):
-            modname = signode["module"]
-            fullname = signode["fullname"]
+            # If the Python signature could not be parsed by Sphinx, `module`
+            # and `fullname` won't be set. Such signatures should be gracefully
+            # ignored.
+            modname = signode.get("module", False)
+            if modname is False:
+                continue
+            fullname = signode.get("fullname", False)
+            if fullname is False:
+                continue
             symbol = (modname + "." if modname else "") + fullname
             if nonodeid and signode["ids"]:
                 orig_node_id = signode["ids"][0]
@@ -116,9 +123,9 @@ def strip_object_entry_node_id(existing_node_id: str, object_id: str):
                         if new_entry[2] == orig_node_id:
                             new_entry[2] = ""
                         new_entries.append(tuple(new_entry))
-                    cast(docutils.nodes.Element, self.indexnode)["entries"] = (
-                        new_entries
-                    )
+                    cast(docutils.nodes.Element, self.indexnode)[
+                        "entries"
+                    ] = new_entries
 
     PyObject.after_content = after_content  # type: ignore[assignment]
 
diff --git a/sphinx_immaterial/apidoc/python/parameter_objects.py b/sphinx_immaterial/apidoc/python/parameter_objects.py
index 8543e786..3c8bce8a 100644
--- a/sphinx_immaterial/apidoc/python/parameter_objects.py
+++ b/sphinx_immaterial/apidoc/python/parameter_objects.py
@@ -469,16 +469,22 @@ def after_content(self: PyObject) -> None:
         obj_desc = cast(
             sphinx.addnodes.desc_content, getattr(self, "contentnode")
         ).parent
-        signodes = obj_desc.children[:-1]
+        signodes = cast(list[sphinx.addnodes.desc_signature], obj_desc.children[:-1])
 
         noindex = "noindex" in self.options
 
         symbols = []
-        for signode in cast(List[docutils.nodes.Element], signodes):
-            modname = signode["module"]
-            fullname = signode["fullname"]
+        valid_signodes: list[sphinx.addnodes.desc_signature] = []
+        for signode in signodes:
+            modname = signode.get("module", False)
+            if modname is False:
+                continue
+            fullname = signode.get("fullname", False)
+            if fullname is False:
+                continue
             symbol = (modname + "." if modname else "") + fullname
             symbols.append(symbol)
+            valid_signodes.append(signode)
 
         if not symbols:
             return
@@ -492,7 +498,7 @@ def after_content(self: PyObject) -> None:
         _cross_link_parameters(
             directive=self,
             app=self.env.app,
-            signodes=cast(List[sphinx.addnodes.desc_signature], signodes),
+            signodes=valid_signodes,
             content=getattr(self, "contentnode"),
             symbols=function_symbols,
             noindex=noindex,
diff --git a/sphinx_immaterial/apidoc/python/synopses.py b/sphinx_immaterial/apidoc/python/synopses.py
index b7f262af..c155b1b6 100644
--- a/sphinx_immaterial/apidoc/python/synopses.py
+++ b/sphinx_immaterial/apidoc/python/synopses.py
@@ -21,15 +21,15 @@ def _add_synopsis(
         obj = self.objects.get(name)
         if obj is None:
             return
-        refnode["reftitle"] = (
-            object_description_options.format_object_description_tooltip(
-                env,
-                object_description_options.get_object_description_options(
-                    env, self.name, obj.objtype
-                ),
-                base_title=name,
-                synopsis=self.data["synopses"].get(name),
-            )
+        refnode[
+            "reftitle"
+        ] = object_description_options.format_object_description_tooltip(
+            env,
+            object_description_options.get_object_description_options(
+                env, self.name, obj.objtype
+            ),
+            base_title=name,
+            synopsis=self.data["synopses"].get(name),
         )
 
     orig_resolve_xref = PythonDomain.resolve_xref
@@ -100,8 +100,12 @@ def after_content(self: PyObject) -> None:
 
         symbols = []
         for signode in cast(List[docutils.nodes.Element], signodes):
-            modname = signode["module"]
-            fullname = signode["fullname"]
+            modname = signode.get("module", False)
+            if modname is False:
+                continue
+            fullname = signode.get("fullname", False)
+            if fullname is False:
+                continue
             symbol = (modname + "." if modname else "") + fullname
             symbols.append(symbol)
 
diff --git a/tests/python_object_ids_test.py b/tests/python_object_ids_test.py
index 98871639..c9c9fb57 100644
--- a/tests/python_object_ids_test.py
+++ b/tests/python_object_ids_test.py
@@ -25,3 +25,25 @@ def test_nonodeid(immaterial_make_app):
 
     assert nodes[0]["ids"] == ["Bar"]
     assert nodes[1]["ids"] == []
+
+
+def test_invalid_signature(immaterial_make_app):
+    app = immaterial_make_app(
+        files={
+            "index.rst": """
+.. py:function:: bar(
+"""
+        },
+    )
+
+    app.build()
+
+    assert not app._warning.getvalue()
+
+    doc = app.env.get_and_resolve_doctree("index", app.builder)
+
+    nodes = list(doc.findall(condition=sphinx.addnodes.desc_signature))
+
+    assert len(nodes) == 1
+
+    assert nodes[0]["ids"] == []