diff --git a/COPYING b/COPYING
index 3eca3450..99f7245c 100644
--- a/COPYING
+++ b/COPYING
@@ -5,6 +5,7 @@ Some included files and subdirectories are licensed with an MIT license, not
 with the GPL. These are:
 
 * engines/LuaCommon/lualib/package.lua (as described in that file)
+* engines/LuaCommon/lualib/strict.lua (as described in that file)
 * engines/LuaCommon/lualib/luabit/ (as described in readme.txt in that directory)
 * engines/LuaCommon/lualib/ustring/ (as described in README in that directory)
 * engines/LuaStandalone/binaries/ (as described in README in that directory)
diff --git a/includes/engines/LuaCommon/lualib/strict.lua b/includes/engines/LuaCommon/lualib/strict.lua
new file mode 100644
index 00000000..5655798a
--- /dev/null
+++ b/includes/engines/LuaCommon/lualib/strict.lua
@@ -0,0 +1,31 @@
+-- strict.lua (fork of http://www.lua.org/extras/5.1/strict.lua)
+-- checks uses of undeclared global variables
+-- All global variables must be 'declared' through a regular assignment
+-- (even assigning nil will do) in a main chunk before being used
+-- anywhere or assigned to inside a function.
+-- distributed under the Lua license: http://www.lua.org/license.html
+-- Scribunto modifications:
+-- * exempt arg, used by require()
+-- * remove what(), since debug.getinfo isn't allowed
+
+local error, rawset, rawget = error, rawset, rawget
+
+local mt = getmetatable(_G)
+if mt == nil then
+  mt = {}
+  setmetatable(_G, mt)
+end
+
+mt.__newindex = function (t, n, v)
+  if n ~= "arg" then
+    error("assign to undeclared variable '"..n.."'", 2)
+  end
+  rawset(t, n, v)
+end
+
+mt.__index = function (t, n)
+  if n ~= "arg" then
+    error("variable '"..n.."' is not declared", 2)
+  end
+  return rawget(t, n)
+end
diff --git a/tests/phpunit/engines/LuaCommon/CommonTests.lua b/tests/phpunit/engines/LuaCommon/CommonTests.lua
index 80fd5f85..b6bece2b 100644
--- a/tests/phpunit/engines/LuaCommon/CommonTests.lua
+++ b/tests/phpunit/engines/LuaCommon/CommonTests.lua
@@ -166,6 +166,22 @@ function test.noLeaksViaPackageLoaded()
 	return 'ok'
 end
 
+function test.strictGood()
+	require( 'strict' )
+	local foo = "bar"
+	return foo
+end
+
+function test.strictBad1()
+	require( 'strict' )
+	return bar
+end
+
+function test.strictBad2()
+	require( 'strict' )
+	bar = "foo"
+end
+
 test.loadData = {}
 
 function test.loadData.get( ... )
@@ -300,6 +316,19 @@ return testframework.getTestProvider( {
 	  expect = { 'ok' },
 	},
 
+	{ name = 'strict on good code raises no errors',
+		func = test.strictGood,
+		expect = { 'bar' },
+	},
+	{ name = 'strict on code reading from a global errors',
+		func = test.strictBad1,
+		expect = "variable 'bar' is not declared",
+	},
+	{ name = 'strict on code setting from a global errors',
+		func = test.strictBad2,
+		expect = "assign to undeclared variable 'bar'",
+	},
+
 	{ name = 'mw.loadData, returning non-table',
 	  func = mw.loadData, args = { 'Module:CommonTests-data-fail1' },
 	  expect = "Module:CommonTests-data-fail1 returned string, table expected",