ClassCastException when getting available transitions in JbpmService

The JbpmService declares a method getAvailableTransitions() which returns a generic List of String.
The service code uses under the hood the getAvailableTransitions() method of the TaskInstance jBPM class, which returns a non generic List containing objects of type Transition.
Unfortunately, the code casts the result of that operation as a Serializable, and further as a generic List of String.
The problem occurs at runtime, the execution falls into a ClassCastException when I try to iterate through the List of objects assumed as String, which actually contains objects of type Transition...