diff --git a/spine/post/metric/match.py b/spine/post/metric/match.py
index 5530eb47..c18043b8 100644
--- a/spine/post/metric/match.py
+++ b/spine/post/metric/match.py
@@ -37,6 +37,7 @@ def __init__(self, fragment=None, particle=None, interaction=None,
         configs = {'fragment': fragment, 'particle': particle,
                    'interaction': interaction}
         self.matchers = {}
+        self.keys = {}
         for key, cfg in configs.items():
             if cfg is not None and cfg != False:
                 # Initialize the matcher