diff --git a/changelogs/fragments/185-core-modules.yml b/changelogs/fragments/185-core-modules.yml
new file mode 100644
index 00000000..456adcd1
--- /dev/null
+++ b/changelogs/fragments/185-core-modules.yml
@@ -0,0 +1,2 @@
+bugfixes:
+  - "Fix ``namespace`` extraction for ansible-core modules (https://github.com/ansible-community/antsibull-changelog/issues/184, https://github.com/ansible-community/antsibull-changelog/pull/185)."
diff --git a/src/antsibull_changelog/plugins.py b/src/antsibull_changelog/plugins.py
index b71d3325..9369cf70 100644
--- a/src/antsibull_changelog/plugins.py
+++ b/src/antsibull_changelog/plugins.py
@@ -159,7 +159,7 @@ def jsondoc_to_metadata(  # pylint: disable=too-many-arguments
             docs = entrypoints["main"]
     if category == "plugin" and plugin_type == "module":
         if is_ansible_core_2_13:
-            last_dot = name.rindex(".")
+            last_dot = name.rfind(".")
             if last_dot >= 0:
                 namespace = name[:last_dot]
                 name = name[last_dot + 1 :]
@@ -400,6 +400,7 @@ def _load_plugins_2_13(
                         plugin_name,
                         plugin_data,
                         category=category[:-1],
+                        is_ansible_core_2_13=True,
                     )
                     plugins_data[category][plugin_type][
                         processed_data["name"]