When trying to migrate a mailbox to Exchange Online using the Transporter, the following error was returned when it tried to determine the source mailbox size:

Object reference not set to an instance of an object

After some investigation it turned out the account we were logged into the migration console with did not have the appropriate access to the mailbox we were trying to migrate. After granting the migration account full control of the user’s mailbox we were able to successfully migrate it.

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