diff --git a/src/calng/SimpleAssembler.py b/src/calng/SimpleAssembler.py
index dbd191fb6968b08dba54641457c557f5c3ed37aa..5081e3ad8f7160ef4dde3b59a9d349f7b87820e3 100644
--- a/src/calng/SimpleAssembler.py
+++ b/src/calng/SimpleAssembler.py
@@ -182,7 +182,7 @@ class SimpleAssembler(TrainMatcher.TrainMatcher):
         self._buffered_status_update.set("rate", self._rate_tracker.get())
         self.set(self._buffered_status_update)
 
-    @functools.lru_cache
+    @functools.lru_cache()
     def _source_to_index(self, source):
         # note: cache means warning only shows up once (also not performance-critical)
         # TODO: allow user to inspect, modify the mapping