diff --git a/pretty.lua b/pretty.lua
index 0098a8e..2ba1331 100644
--- a/pretty.lua
+++ b/pretty.lua
@@ -611,10 +611,16 @@ local KNOWN_OPTIONS = {
     recursion           = { type = 'string',  default = 'ignore', accepted = {['ignore'] = true, ['marked'] = true} },
 }
 
-local function ensure_that_all_options_are_known (options)
-    assert(type(options) == 'table')
+local function ensure_that_all_options_are_known (input_options)
+    -- Goes through all the given options, throws error if one is unknown, gives
+    -- warning if debug or experimental. Creates a clone of the given table, to
+    -- avoid defaults leaking.
+
+    -- Error check that options were table
+    assert(type(input_options) == 'table')
+
     -- Error check options
-    for option_name, option_value in pairs(options) do
+    for option_name, option_value in pairs(input_options) do
         if not KNOWN_OPTIONS[option_name] then
             error(('[pretty]: Unknown option: %s. Was given value %s (%s)'):format(option_name, option_value, type(option_value)), 2)
         elseif type(option_value) ~= KNOWN_OPTIONS[option_name].type then
@@ -626,14 +632,19 @@ local function ensure_that_all_options_are_known (options)
             print(('[pretty]: Using %s option "%s".\n    Please note that this option may change at any time. It is not stable,\n    not tested, and may indeed break or be removed without warning.'):format(KNOWN_OPTIONS[option_name].debug, option_name))
         end
     end
+
+    -- Create output options
+    local output_options = {}
     -- Assign default values
     for option_name, option_info in pairs(KNOWN_OPTIONS) do
-        if options[option_name] == nil then
-            options[option_name] = option_info.default
+        if input_options[option_name] ~= nil then
+            output_options[option_name] = input_options[option_name]
+        else
+            output_options[option_name] = option_info.default
         end
     end
-    -- Returns options
-    return options
+    -- Returns input_options
+    return output_options
 end
 
 local function pretty_format (value, options)