botframework-sdk: Conversation.ResumeAsync returns "Operation is not valid due to the current state of the object"

I am trying to resume a conversation after completion of an external even. I’m using Conversation.ResumeAsync to do so, passing in the resumptionCookie and a message. I’m getting the above error in return which is not very helpful. So now I’m stuck trying to find out what it going wrong and why. Would be nice to have some pointers as to where to look!

Exception: System.InvalidOperationException: Operation is not valid due to the current state of the object. at Microsoft.Bot.Builder.Dialogs.Conversation.<>c31.<ResumeAsync>b3_0() at Microsoft.Bot.Builder.Dialogs.Internals.ReactiveDialogTask.<Microsoft-Bot-Builder-Dialogs-Internals-IPostToBot-PostAsync>d31.MoveNext() — End of stack trace from previous location where exception was thrown — at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.Bot.Builder.Dialogs.Internals.ExceptionTranslationDialogTask.<Microsoft-Bot-Builder-Dialogs-Internals-IPostToBot-PostAsync>d21.MoveNext() — End of stack trace from previous location where exception was thrown — at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.Bot.Builder.Dialogs.Internals.LocalizedDialogTask.<Microsoft-Bot-Builder-Dialogs-Internals-IPostToBot-PostAsync>d__21.MoveNext() — End of stack trace from previous location where exception was thrown — at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.Bot.Builder.Dialogs.Internals.ScoringDialogTask1.<Microsoft-Bot-Builder-Dialogs-Internals-IPostToBot-PostAsync>d__31.MoveNext() — End of stack trace from previous location where exception was thrown — at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.Bot.Builder.Dialogs.Internals.PersistentDialogTask.<Microsoft-Bot-Builder-Dialogs-Internals-IPostToBot-PostAsync>d31.MoveNext() — End of stack trace from previous location where exception was thrown — at Microsoft.Bot.Builder.Dialogs.Internals.PersistentDialogTask.<Microsoft-Bot-Builder-Dialogs-Internals-IPostToBot-PostAsync>d31.MoveNext() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.Bot.Builder.Dialogs.Internals.SerializingDialogTask.&lt;Microsoft-Bot-Builder-Dialogs-Internals-IPostToBot-PostAsync&gt;d__41.MoveNext() — End of stack trace from previous location where exception was thrown — at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.Bot.Builder.Dialogs.Internals.PostUnhandledExceptionToUserTask.<Microsoft-Bot-Builder-Dialogs-Internals-IPostToBot-PostAsync>d__51.MoveNext()