The composition of software units has been one of the main research topics in computer science. This paper addresses the composition validation problem evolving in this context. It focuses on the composition for a certain kind of units called aspects. Aspects are a new concept which is introduced by aspect-oriented programming aiming at a better separation of concerns. Cross-cutting code is captured and localised in these aspects. Some of the cross-cutting features which are expressed in aspects cannot be woven with other features into the same application since two features could be mutually exclusive. With a growing number of aspects, manual control of these dependencies becomes error-prone or even impossible. We show how assertions can be useful in this respect to support the software developer.