Getting "lower-bounds (failed: Unbound type constructor Yojson.Basic.t)" when publishing to Opam

I keep seeing "lower-bounds (failed: Unbound type constructor Yojson.Basic.t)" when trying to publish to opam.

When I clicked the link and read the content, I see the below:

# 59 |       web_config -> string -> string -> string -> int * Yojson.Basic.t
#                                                              ^^^^^^^^^^^^^^
# Error: Unbound type constructor Yojson.Basic.t

It's referencing my .mli file below:

module Database :
  sig
    type meta_data = {
      fs : in_channel;
      db_type : int;
      db_column : int;
      db_year : int;
      db_month : int;
      db_day : int;
      ipv4_db_count : Stdint.uint32;
      ipv4_base_addr : Stdint.uint32;
      ipv6_db_count : Stdint.uint32;
      ipv6_base_addr : Stdint.uint32;
      ipv4_index_base_addr : Stdint.uint32;
      ipv6_index_base_addr : Stdint.uint32;
      ipv4_column_size : Stdint.uint32;
      ipv6_column_size : Stdint.uint32;
    }
    type ip2location_record = {
      country_short : string;
      country_long : string;
      region : string;
      city : string;
      isp : string;
      latitude : float;
      longitude : float;
      domain : string;
      zip_code : string;
      time_zone : string;
      net_speed : string;
      idd_code : string;
      area_code : string;
      weather_station_code : string;
      weather_station_name : string;
      mcc : string;
      mnc : string;
      mobile_brand : string;
      elevation : float;
      usage_type : string;
      address_type : string;
      category : string;
    }
    exception Ip2location_exception of string
    val get_api_version : string
    val open_db : string -> meta_data
    val close_db : meta_data -> unit
    val query : meta_data -> string -> ip2location_record
  end
module Web_service :
  sig
    type web_config = {
      api_key : string;
      api_package : string;
      use_ssl : bool;
    }
    exception Ip2location_exception of string
    val open_ws : string -> string -> bool -> web_config
    val lookup :
      web_config -> string -> string -> string -> int * Yojson.Basic.t
    val get_credit : web_config -> int * Yojson.Basic.t
  end

I have no idea what I'm missing now. Any pointers would be great.

This is an issue with your opam file which is missing the information that your package is not compatible with earlier versions of yojson. Typically, the type Yojson.Base.t was introduced as an alias of Yojson.Base.json in version 1.6.0 .