diff --git a/json.lua b/json.lua
index 205dfb4..19f1571 100644
--- a/json.lua
+++ b/json.lua
@@ -54,14 +54,13 @@ local function encode_table(val, stack)
     -- Treat as array -- check keys are valid and it is not sparse
     local n = 0
     for k in pairs(val) do
-      local t = type(k)
-      if t ~= "number" then
-        error("unexpected key of type '" .. t .. "' in array")
+      if type(k) ~= "number" then
+        error("invalid table: mixed key types")
       end
       n = n + 1
     end
     if n ~= #val then
-      error("unexpected sparse array")
+      error("invalid table: sparse array")
     end
     -- Encode
     for i, v in ipairs(val) do
@@ -73,9 +72,8 @@ local function encode_table(val, stack)
   else
     -- Treat as an object
     for k, v in pairs(val) do
-      local t = type(k)
-      if t ~= "string" then
-        error("unexpected key of type '" .. t .. "' in object")
+      if type(k) ~= "string" then
+        error("invalid table: mixed key types")
       end
       table.insert(res, encode(k, stack) .. ":" .. encode(v, stack))
     end