diff --git a/Lib9c/ValidatorDelegation/AbstainHistory.cs b/Lib9c/ValidatorDelegation/AbstainHistory.cs
index c90a987361..430d2c78cd 100644
--- a/Lib9c/ValidatorDelegation/AbstainHistory.cs
+++ b/Lib9c/ValidatorDelegation/AbstainHistory.cs
@@ -78,7 +78,7 @@ public List<PublicKey> FindToSlashAndAdd(IEnumerable<PublicKey> abstainList, lon
                 }
             }
 
-            foreach (var history in History)
+            foreach (var history in History.ToArray())
             {
                 history.Value.RemoveAll(abstainHeight => abstainHeight < lowerBound);
                 if (history.Value.Count == 0)