diff --git a/script/json.lua b/script/json.lua
index eaccb43e0..6ae79aef6 100644
--- a/script/json.lua
+++ b/script/json.lua
@@ -18,6 +18,7 @@ local huge = math.huge
 local tiny = -huge
 
 local utf8_char
+local utf8_len
 local math_type
 
 if _VERSION == "Lua 5.1" or _VERSION == "Lua 5.2" then
@@ -51,6 +52,7 @@ if _VERSION == "Lua 5.1" or _VERSION == "Lua 5.2" then
     end
 else
     utf8_char = utf8.char
+    utf8_len = utf8.len
     math_type = math.type
 end
 
@@ -118,7 +120,26 @@ encode_map["nil"] = function ()
     return "null"
 end
 
+local function replace_invalid_utf8(v)
+    local result = {}
+    local start = 1
+    while true do
+        local _, invalid = utf8_len(v, start)
+        if not invalid then
+            result[#result+1] = string_sub(v, start)
+            break
+        end
+        result[#result+1] = string_sub(v, start, invalid - 1)
+        result[#result+1] = utf8_char(0xfffd)
+        start = invalid + 1
+    end
+    return table_concat(result)
+end
+
 local function encode_string(v)
+    if utf8_len and not utf8_len(v) then
+        v = replace_invalid_utf8(v)
+    end
     return string_gsub(v, '[%z\1-\31\\"]', encode_escape_map)
 end
 
diff --git a/test/other/init.lua b/test/other/init.lua
index fbe9d923b..3f5717ec9 100644
--- a/test/other/init.lua
+++ b/test/other/init.lua
@@ -1 +1,2 @@
 --require 'other.filewatch'
+require 'other.json'
diff --git a/test/other/json.lua b/test/other/json.lua
new file mode 100644
index 000000000..322171efd
--- /dev/null
+++ b/test/other/json.lua
@@ -0,0 +1,25 @@
+local json = require 'json'
+
+local replacement = utf8.char(0xfffd)
+
+local function assertEncoding(value, expected)
+    local encoded = json.encode(value)
+    assert(utf8.len(encoded))
+    assert(encoded == expected, ('expected %q, got %q'):format(expected, encoded))
+end
+
+assertEncoding('plain text', '"plain text"')
+assertEncoding('é中文', '"é中文"')
+assertEncoding('\x80', '"' .. replacement .. '"')
+assertEncoding('\xff', '"' .. replacement .. '"')
+assertEncoding('\xc2A', '"' .. replacement .. 'A"')
+assertEncoding('\xe2\x82', '"' .. replacement .. replacement .. '"')
+assertEncoding('[^%w_\x80-\xff]', '"[^%w_' .. replacement .. '-' .. replacement .. ']"')
+assertEncoding({ ['\x80'] = 'value' }, '{"' .. replacement .. '":"value"}')
+
+assert(json.decode(json.encode('\x80')) == replacement)
+
+local jsonb = require 'json-beautify'
+local beautified = jsonb.beautify({ value = '\x80' })
+assert(utf8.len(beautified))
+assert(beautified:find(replacement, 1, true))