How to fix this unchecked assignment warning?

var blks = ReflectionHelper.getClasses("PACKAGE_NAME");
var blockRta = RuntimeTypeAdapterFactory.of(Block.class);
for (var c : blks) {
    if (Block.class.isAssignableFrom(c)) {
        blockRta.registerSubtype(c);
    }
}

I got Warning:(31, 46) Unchecked assignment: 'java.lang.Class' to 'java.lang.Class<? extends PACKAGE_NAME.Block>' warning on the line blockRta.registerSubtype(c);, but I can't figure out how to fix that without supressing it.

ReflectionHelper.getClasses is a static method to get all the classes in that package name, and its return type is Class[]. Block is an interface. RuntimeTypeAdapterFactory is a class in gson extra, and its source code can be viewed here.