diff --git a/src/cattr/converters.py b/src/cattr/converters.py
index 2a618b96..6c7a7c7c 100644
--- a/src/cattr/converters.py
+++ b/src/cattr/converters.py
@@ -138,7 +138,7 @@ def __init__(
         # Unions are instances now, not classes. We use a different registry.
         self._union_registry = {}
 
-    def unstructure(self, obj):
+    def unstructure(self, obj: Any) -> Any:
         return self._unstructure_func.dispatch(obj.__class__)(obj)
 
     @property