Error when delete forum

I have tried to delete a forum but I get this error.

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Unable to create a constant value of type 'Nop.Core.Domain.Forums.ForumTopic'. Only primitive types ('such as Int32, String, and Guid') are supported in this context.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NotSupportedException: Unable to create a constant value of type 'Nop.Core.Domain.Forums.ForumTopic'. Only primitive types ('such as Int32, String, and Guid') are supported in this context.

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What is wrong and how can I delete the forum. There are no threads in the forum.

You're right. Thanks for reporting. I'll have a look at this issue in the near time. Not sure but seems that this issue appeared after upgrading to Entity Framework 4.3

Fixed. Please see changeset 28a7a67cead3