The effects of heterogeneities (weak zones in particular) in adhesive joints and their importance on overall fracture properties are relatively unknown, but doubtlessly they may be crucial in many applications. Using a model heterogeneous adhesive bond, represented by a given mixture of regions of strong and weak adhesion, we have studied the influence of interface variability on overall fracture energy (global energy release rate). Adopting the original Griffith-Irwin arguments, we have employed a simple, fracture scaling law. By varying relative proportions of the weak and strong interfaces, a nonlinear evolution of fracture energy was observed. This was contrary to expectations, cf. rules of mixtures. Inspired by rheological models (Maxwell, Voigt/Kelvin, etc.), an appropriate model was found empirically.