diff --git a/src/pypendency/container.py b/src/pypendency/container.py
index a392b39..102ab94 100644
--- a/src/pypendency/container.py
+++ b/src/pypendency/container.py
@@ -28,7 +28,7 @@ def __init__(self, definitions: List[Definition]):
             definition.identifier: definition
             for definition in definitions
         }
-        self._tags_mapping: Dict[Tag, List[str]] = {}
+        self._tags_mapping: Dict[Tag, Set[str]] = {}
 
     def resolve(self) -> None:
         if self.is_resolved():