Weird issue with inheritance and interfaces

Function Main()
	Local a:= New MyExtendedClass
	SimpleFunction(a)
end


Interface MyBaseInterface
	Method DoSomething:String(value1:Int)
End

Interface MyExtendedInterface Extends MyBaseInterface
	Method DoSomethingMore:String(value2:Int)
End

Class MyBaseClass implements MyBaseInterface
	Method DoSomething:String(value1:Int)
		Print "Something being done"
		Return "Hello!"
	end
End

Class MyExtendedClass extends MyBaseClass implements MyExtendedInterface 
	Method DoSomethingMore:String(value2:Int)
		Return "The number is " + value2
	end
End

Function SimpleFunction(a:MyExtendedInterface)
	a.DoSomething(12)
End