diff --git a/src/dependency_injector/wiring.py b/src/dependency_injector/wiring.py
index b1f01622..2088484f 100644
--- a/src/dependency_injector/wiring.py
+++ b/src/dependency_injector/wiring.py
@@ -605,7 +605,7 @@ def _locate_dependent_closing_args(provider: providers.Provider) -> Dict[str, pr
         if not arg.args and isinstance(arg, providers.Resource):
             return {str(id(arg)): arg}
         else:
-            closing_deps += _locate_dependent_closing_args(arg)
+            closing_deps |= _locate_dependent_closing_args(arg)
     return closing_deps