diff --git a/lib/write-type.js b/lib/write-type.js
index 94a10ab..67b5657 100644
--- a/lib/write-type.js
+++ b/lib/write-type.js
@@ -31,6 +31,7 @@ function getWriteType(options) {
   var bin = binarraybuffer ? bin_arraybuffer : bin_buffer;
   var usemap = HAS_MAP && options && options.usemap;
   var map = usemap ? map_to_map : obj_to_map;
+  var strip_undef = options && options.strip_undef;
 
   var writeType = {
     "boolean": bool,
@@ -229,6 +230,12 @@ function getWriteType(options) {
   // map 32 -- 0xdf
   function obj_to_map(encoder, value) {
     var keys = Object.keys(value);
+
+    if (strip_undef) {
+      // skip encoding keys if their value is undefined
+      keys = keys.filter(function(key) { return value[key] !== undefined; });
+    }
+
     var length = keys.length;
     var type = (length < 16) ? (0x80 + length) : (length <= 0xFFFF) ? 0xde : 0xdf;
     token[type](encoder, length);