Reflection - getDeclaredMethod - Parametertyp expects interface

I need to call a method via reflection. I'm able to find and call the class and the method.
But the argument/parameter is an interface, which I initialize with a class which implements this interface and I call the method with "it".

ParameterType: interface de.myapp.IParameters
GenericParameterType: interface de.myapp.IParameters

IParameters arguments = new DefaultParameter(x,y,z);
...
Object result = methodWithInterfaceparameter.invoke(myClass, arguments );

Exception:

java.lang.IllegalArgumentException: object is not an instance of declaring class

Any help is appreciated.