Cause
This error occurs if your application code attempts to use a key in a dictionary that is not an instance of the class, or any of its subclasses, used as a key in the dictionary.
Objects that can act as keys in a dictionary must be defined in the schema, by specifying the class of keys that the dictionary can use.
Action
Redefine the dictionary definition to the correct class, or change the application code that resulted in this error.