diff --git a/kong/clustering/rpc/callbacks.lua b/kong/clustering/rpc/callbacks.lua
index 2935cbab5a54..05a0a89c45d0 100644
--- a/kong/clustering/rpc/callbacks.lua
+++ b/kong/clustering/rpc/callbacks.lua
@@ -9,6 +9,7 @@ local _M = {}
 local _MT = { __index = _M, }
 
 
+local cjson = require("cjson.safe")
 local utils = require("kong.clustering.rpc.utils")
 
 
@@ -22,6 +23,9 @@ function _M.new()
     capabilities_list = {}, -- updated as register() is called
   }
 
+  -- it should always be an array when json encoding
+  setmetatable(self.capabilities_list, cjson.array_mt)
+
   return setmetatable(self, _MT)
 end