This creates a standard horizontal/vertical state change diagram:
@startuml
Hide Empty Description
Bob --> Alice : hello
Alice --> Bob : bye
@enduml

If I want to add a description line to one of the states like this:
@startuml
Hide Empty Description
Bob --> Alice : hello
Bob : Test
Alice --> Bob : bye
@enduml
Why does it turn into a class diagram like this:

How do I force this to be a state machine diagram and not a class diagram?