The Australian Aboriginal language Jingulu exhibits an affix-triggered vowel harmony pattern that deviates from usual typological predictions. High vowels in a handful of specific Jingulu affixes harmonize unlimited strings of root vowels, but cannot affect vowels beyond a barrier marked by [+high], the exact feature value for which harmony demands agreement. Although weak-trigger harmony systems are widely attested in the literature, the blocking effect in Jingulu is, to my knowledge, currently unattested elsewhere. The present analysis captures the phenomenon in Optimality Theory using a licensing constraint to drive harmony, and posits that the blocking effect is a consequence of Lexicon Optimization for the Obligatory Contour Principle in tandem with a constraint mandating identical feature-sharing relations between input and output mappings. A string of adjacent vowels with the same height specification is assumed to share a single height feature underlyingly, such that the licensing constraint targets individual height features rather than segments.