I continue to receive a runtime error message each time I try t run a backtest. It will run for a month or so and then give me this error. Anyone know what the problem is?

Error Message:Runtime Error: In Scheduled Event 'EveryDay: 11', Collection was modified; enumeration operation may not execute.
Stacktrace:QuantConnect.Scheduling.ScheduledEventException: In Scheduled Event 'EveryDay: 11', ---> System.InvalidOperationException: Collection was modified; enumeration operation may not execute. at System.Collections.Generic.Dictionary`2+Enumerator[TKey,TValue].MoveNext () [0x00013] in <b0e1ad7573a24fd5a9f2af9595e677e7>:0 at QuantConnect.Algorithm.CSharp.TestAlgorithm.CleanContracts () [0x00087] in <ef784a9ac7e7494cb624457bd36ee0f0>:0 at QuantConnect.Algorithm.CSharp.TestAlgorithm.Schedule1Fired () [0x000b0] in <ef784a9ac7e7494cb624457bd36ee0f0>:0 at QuantConnect.Scheduling.ScheduleManager+<>c__DisplayClass14_0.<On>b__0 (System.String name, System.DateTime time) [0x00000] in <98c88ae28c7a4d6394051ec2103eb0d3>:0 at QuantConnect.Scheduling.ScheduledEvent.OnEventFired (System.DateTime triggerTime) [0x00027] in <98c88ae28c7a4d6394051ec2103eb0d3>:0 --- End of inner exception stack trace ---

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