diff --git a/changelog.d/8873.doc b/changelog.d/8873.doc
new file mode 100644
index 000000000..0c2a043bd
--- /dev/null
+++ b/changelog.d/8873.doc
@@ -0,0 +1 @@
+Fix an error in the documentation for the SAML username mapping provider.
diff --git a/docs/sso_mapping_providers.md b/docs/sso_mapping_providers.md
index dee53b5d4..c13b4f715 100644
--- a/docs/sso_mapping_providers.md
+++ b/docs/sso_mapping_providers.md
@@ -116,11 +116,13 @@ comment these options out and use those specified by the module instead.
 
 A custom mapping provider must specify the following methods:
 
-* `__init__(self, parsed_config)`
+* `__init__(self, parsed_config, module_api)`
    - Arguments:
      - `parsed_config` - A configuration object that is the return value of the
        `parse_config` method. You should set any configuration options needed by
        the module here.
+     - `module_api` - a `synapse.module_api.ModuleApi` object which provides the
+       stable API available for extension modules.
 * `parse_config(config)`
     - This method should have the `@staticmethod` decoration.
     - Arguments: