Collection modified exception while getting message count for a label

[alimbada]

System.InvalidOperationException was unhandled
Message=Collection was modified; enumeration operation may not execute.
Source=mscorlib
StackTrace:
at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
at System.Collections.Generic.List1.Enumerator.MoveNextRare() at System.Collections.Generic.List1.Enumerator.MoveNext()
at System.Linq.Enumerable.WhereSelectListIterator2.MoveNext() at System.Linq.Enumerable.Sum(IEnumerable1 source)
at System.Linq.Enumerable.Sum[TSource](IEnumerable1 source, Func2 selector)
at Inbox2.Framework.VirtualMailBox.Entities.LabelsContainer.GetCountAsync(Object state) in C:\dev\inbox2_desktop\Code\Client\Inbox2\Framework\VirtualMailBox\Entities\LabelsContainer.cs:line 60
at System.Threading.ExecutionContext.runTryCode(Object userData)
at System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading._ThreadPoolWaitCallback.PerformWaitCallbackInternal(_ThreadPoolWaitCallback tpWaitCallBack)
at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback(Object state)