diff --git a/lua-parser/pp.lua b/lua-parser/pp.lua
index 649db38..5eb1ae6 100644
--- a/lua-parser/pp.lua
+++ b/lua-parser/pp.lua
@@ -18,7 +18,7 @@ end
 local function fixed_string (str)
   local new_str = ""
   for i=1,string.len(str) do
-    char = string.byte(str, i)
+    local char = string.byte(str, i)
     if char == 34 then new_str = new_str .. string.format("\\\"")
     elseif char == 92 then new_str = new_str .. string.format("\\\\")
     elseif char == 7 then new_str = new_str .. string.format("\\a")
diff --git a/test-pp.lua b/test-pp.lua
new file mode 100644
index 0000000..505d843
--- /dev/null
+++ b/test-pp.lua
@@ -0,0 +1,11 @@
+local pp = require "lua-parser.pp"
+local saved_char = rawget(_G, "char")
+local sentinel = {}
+_G.char = sentinel
+local ast = {{ tag = "Return", { tag = "String", "hello" } }}
+assert(pp.tostring(ast) == '{ `Return{ `String "hello" } }')
+assert(_G.char == sentinel, "pretty printing changed global char")
+_G.char = nil
+pp.tostring(ast)
+assert(rawget(_G, "char") == nil, "pretty printing created global char")
+_G.char = saved_char
diff --git a/test.lua b/test.lua
index 944215d..978ccb9 100755
--- a/test.lua
+++ b/test.lua
@@ -1,3 +1,5 @@
+dofile("test-pp.lua")
+
 #!/usr/bin/env lua
 
 local parser = require "lua-parser.parser"