Julia is an untyped imperative programming language designed for scientific computing. Despite being untyped, Julia provides a rich runtime type system that includes features such as inheritance, but lacks the mechanisms to ensure compliance with interfaces. We propose a static type system for a subset of Julia, called Jolt, ruling out functional interface mismatches by synthesizing abstract interfaces from concrete implementations. Jolt can rule out some type errors in existing code without any new annotations, providing additional safety for free.