Hi Yuriy, I guess you subclassed your class from the (wrong) class "ExternalStructure" but in the guide it is "FFIExternalStructure". Make sure to accept in the browser: FFIExternalStructure subclass: #MyStructure instanceVariableNames: '' classVariableNames: '' package: 'FFIDemo' instead of ExternalStructure subclass: #MyStructure instanceVariableNames: '' classVariableNames: '' package: 'FFIDemo' to make the class MyStructure a subclass of FFIExternalStructure instead of ExternalStructure. Hope this helps! Bye Torsten