diff --git a/reactor/Core/src/jetbrains/mps/logic/reactor/core/internal/ConstraintsProcessing.kt b/reactor/Core/src/jetbrains/mps/logic/reactor/core/internal/ConstraintsProcessing.kt
index ac71ba03..08e1c2a6 100644
--- a/reactor/Core/src/jetbrains/mps/logic/reactor/core/internal/ConstraintsProcessing.kt
+++ b/reactor/Core/src/jetbrains/mps/logic/reactor/core/internal/ConstraintsProcessing.kt
@@ -204,7 +204,7 @@ internal class ConstraintsProcessing(private var dispatchingFront: Dispatcher.Di
         return SessionTokenImpl(histView, rules, principalState, logicalState.clear())
     }
 
-    fun invalidatedRules(): Set<Any> = invalidatedRulesTags
+    fun invalidatedRules(): Set<Any> = HashSet<Any>(invalidatedRulesTags)
 
     /**
      * Called to update the state with the currently active constraint occurrence.