diff --git a/sandbox.lua b/sandbox.lua
index e8f2d78..66ff7c4 100644
--- a/sandbox.lua
+++ b/sandbox.lua
@@ -1,5 +1,4 @@
 local BASE_ENV = {}
-
 -- Non-safe :
 -- string.rep: can be used to allocate millions of bytes in 1 operation
 -- {set|get}metatable: can be used to modify the metatable of global objects (strings, integers)
@@ -48,6 +47,15 @@ end)
 
 local string_rep = string.rep
 
+local function copy(other)
+  if type(other) ~= 'table' then return other end
+  local c = {}
+  for k,v in pairs(other) do
+    c[copy(k)] = copy(v)
+  end
+  return c
+end
+
 local function cleanup()
   debug.sethook()
   string.rep = string_rep
@@ -60,7 +68,9 @@ local function run(f, options)
 
   local limit = options.limit or 500000
 
-  setfenv(f, BASE_ENV)
+  local env = copy(BASE_ENV)
+
+  setfenv(f, env)
 
   -- I would love to be able to make step greater than 1
   -- (say, 500000) but any value > 1 seems to choke with a simple while true do end
diff --git a/spec/sandbox_spec.lua b/spec/sandbox_spec.lua
index 36b528d..7aa0406 100644
--- a/spec/sandbox_spec.lua
+++ b/spec/sandbox_spec.lua
@@ -38,6 +38,11 @@ describe('sandbox', function()
     assert.equal('hellohello', string.rep('hello', 2))
   end)
 
+  it('should not persist modifying the packages', function()
+    sandbox("string.foo = 1")
+    assert.is_nil(sandbox("return string.foo"))
+  end)
+
 
   describe('when handling infinite loops', function()
 
@@ -54,6 +59,9 @@ describe('sandbox', function()
       assert.no_has_error(function() sandbox("for i=1,100 do end") end)
       assert.has_error(function() sandbox("for i=1,100 do end", {limit = 50}) end)
     end)
+
   end)
 
+
+
 end)