Method toString(), should be ToString().
In the Wiki documents we read:
"The toString function converts an object to its string representation so that it is suitable for display. Although .NET language uses the function ToString, you should type it as toString in order to ensure compatibility between the Bizagi .NET and JEE version. This should not be an issue."
This call gives a very ambiguous error, and in fact an opposite is true. "ToString()" works perfect.