Why is Hans Christian Andersen in jail in thumbliena?

In "Thumbelina" by Hans Christian Andersen, the swallow informs Thumbelina that the Prince of the Fairies knows of Thumbelina's good deeds and he is willing to marry her. This causes Thumbelina to go to the palace and remember how she met the swallow, the prince breaks the spell over her and she becomes a real fairy, obtaining her wings.