UDF_ARGS "attribute_lengths" incorrect value

I try to use User Defined Function lib_mysqludf_json from https://github.com/mysqludf/lib_mysqludf_json with MariaDB 10.1.18 and found an incorrect behavior of UDF_ARGS attribute_lengths.

typedef struct st_udf_args

{

...

  unsigned long *attribute_lengths;     /* Length of attribute arguments */

...

} UDF_ARGS;

Example:
When you use SELECT json_object("citizen_id") FROM ...
you will have:

args->attributes[0] = "citizen_id"

args->attribute_lengths[0] = 12

That's correct.

When you use SELECT json_object(citizen_id AS citizen_id) FROM ...

args->attributes[0] = citizen_id

args->attribute_lengths[0] = 10

That's also correct.

But when you use SELECT json_object(citizen_id) FROM ...

args->attributes[0] = citizen_id

args->attribute_lengths[0] = 0

That's incorrect. Correct value of attribute_lengths[0] is 10.

That's not reproducable on MySQL. This is not lib_mysqludf_json incorrect behavior, it just uses standart struct UDF_ARGS.