Unhandled exception on dynamicdata version 8.3.25 onwards

[vkapoorjefco]

On upgrading from version 8.1.1 to 8.3.25 I am getting an exception when i perform a AddOrUpdate on my Sourcecache<string,Order>. This doesnot happen in the earlier version.
System.ArgumentNullException: Value cannot be null. (Parameter 'key')
at System.Collections.Generic.Dictionary2.FindValue(TKey key) at System.Collections.Generic.Dictionary2.TryGetValue(TKey key, TValue& value)
at DynamicData.Kernel.OptionExtensions.Lookup[TValue,TKey](IDictionary2 source, TKey key) in /_/src/DynamicData/Kernel/OptionExtensions.cs:line 178 at DynamicData.Cache.Internal.DistinctCalculator3.<>c__DisplayClass7_0.g__AddValueAction|1(TValue value) in //src/DynamicData/Cache/Internal/DistinctCalculator.cs:line 38
at DynamicData.Cache.Internal.DistinctCalculator3.Calculate(IChangeSet2 changes) in //src/DynamicData/Cache/Internal/DistinctCalculator.cs:line 96
at System.Reactive.Linq.ObservableImpl.Select`2.Selector._.OnNext(TSource value)
--- End of stack trace from previous location ---

[dwcullop]

Would it be possible to create a minimal example that recreates the problem?