diff --git a/src/ConfigSpace/configuration.py b/src/ConfigSpace/configuration.py
index 4874287a..34ab7892 100644
--- a/src/ConfigSpace/configuration.py
+++ b/src/ConfigSpace/configuration.py
@@ -173,11 +173,11 @@ def get_array(self) -> Array[f64]:
         """
         return self._vector
 
-    def __contains__(self, item: object) -> bool:
-        if not isinstance(item, str):
+    def __contains__(self, key: object) -> bool:
+        if not isinstance(key, str):
             return False
 
-        return item in self
+        return key in self.config_space
 
     def __setitem__(self, key: str, value: Any) -> None:
         param = self.config_space[key]
diff --git a/test/test_configuration_space.py b/test/test_configuration_space.py
index 830fca17..22796387 100644
--- a/test/test_configuration_space.py
+++ b/test/test_configuration_space.py
@@ -1233,6 +1233,9 @@ def test_configuration_acts_as_mapping(simple_cs: ConfigurationSpace):
     # Test indexing
     assert config["parent"] == values_dict["parent"]
     assert config["child"] == values_dict["child"]
+    for name in names:
+        assert name in config
+    assert "mouse" not in config
 
     # Test dict methods
     assert set(config.keys()) == set(names)