Интерфейсы не надо просматривать

src/main/kotlin/ru/dbotthepony/kstarbound/io/json/builder/BuilderAdapter.kt

@@ -359,8 +359,10 @@ class BuilderAdapter<T : Any> private constructor(
 			}
 
 			for (parent in input.supertypes) {
-				if (parent.classifier is KClass<*>) {
-					collectDecl(parent.classifier as KClass<*>, output)
+				val classifier = parent.classifier
+
+				if (classifier is KClass<*> && !classifier.java.isInterface) {
+					collectDecl(classifier, output)
 				}
 			}
 		}