ValidationErrorMessageHandler.getModelMessageKey() line 211 returns null.
parentType = beanWrapper.getPropertyType(parentPath);
In this case, given the full path "collection[0].subProperty", the parent path is "collection[0]". beanWrapper.getPropertyType("collection[0]") returns null when the item at collection[0] is null.
This method should be using PathUtils to unwrap the collection type, instead of beanWrapper directly.
Also, springjutsu-validation should really be using spring 4 for the better parameterization handling.