diff --git a/Readme.md b/Readme.md
index 7bfea3e..b09df6a 100644
--- a/Readme.md
+++ b/Readme.md
@@ -218,7 +218,7 @@ form.append( 'my_file', fs.createReadStream('/foo/bar.jpg'), {filename: 'bar.jpg
 ```
 
 #### _Headers_ getHeaders( [**Headers** _userHeaders_] )
-This method adds the correct `content-type` header to the provided array of `userHeaders`.  
+This method adds the correct `content-type` header to the provided array of `userHeaders`.
 
 #### _String_ getBoundary()
 Return the boundary of the formData. By default, the boundary consists of 26 `-` followed by 24 numbers
@@ -348,6 +348,8 @@ axios.post('http://example.com', form, {
 ## Notes
 
 - ```getLengthSync()``` method DOESN'T calculate length for streams, use ```knownLength``` options as workaround.
+- ```getLength(cb)``` will send an error as first parameter of callback if stream length cannot be calculated (e.g. send in custom streams w/o using ```knownLength```).
+- ```submit``` will not add `content-length` if form length is unknown or not calculable.
 - Starting version `2.x` FormData has dropped support for `node@0.10.x`.
 - Starting version `3.x` FormData has dropped support for `node@4.x`.
 
diff --git a/lib/form_data.js b/lib/form_data.js
index cf836b0..18dc819 100644
--- a/lib/form_data.js
+++ b/lib/form_data.js
@@ -5,6 +5,7 @@ var http = require('http');
 var https = require('https');
 var parseUrl = require('url').parse;
 var fs = require('fs');
+var Stream = require('stream').Stream;
 var mime = require('mime-types');
 var asynckit = require('asynckit');
 var populate = require('./populate.js');
@@ -100,8 +101,8 @@ FormData.prototype._trackLength = function(header, value, options) {
     Buffer.byteLength(header) +
     FormData.LINE_BREAK.length;
 
-  // empty or either doesn't have path or not an http response
-  if (!value || ( !value.path && !(value.readable && value.hasOwnProperty('httpVersion')) )) {
+  // empty or either doesn't have path or not an http response or not a stream
+  if (!value || ( !value.path && !(value.readable && value.hasOwnProperty('httpVersion')) && !(value instanceof Stream))) {
     return;
   }
 
@@ -456,13 +457,15 @@ FormData.prototype.submit = function(params, cb) {
 
   // get content length and fire away
   this.getLength(function(err, length) {
-    if (err) {
+    if (err && err !== 'Unknown stream') {
       this._error(err);
       return;
     }
 
     // add content length
-    request.setHeader('Content-Length', length);
+    if (length) {
+      request.setHeader('Content-Length', length);
+    }
 
     this.pipe(request);
     if (cb) {
diff --git a/test/integration/test-form-get-length-sync.js b/test/integration/test-form-get-length-sync.js
index e212a76..9901c7e 100644
--- a/test/integration/test-form-get-length-sync.js
+++ b/test/integration/test-form-get-length-sync.js
@@ -2,6 +2,7 @@ var common = require('../common');
 var assert = common.assert;
 var FormData = require(common.dir.lib + '/form_data');
 var fs = require('fs');
+var Readable = require('stream').Readable;
 
 (function testGetLengthSync() {
   var fields = [
@@ -73,3 +74,34 @@ var fs = require('fs');
 
   assert.equal(expectedLength, calculatedLength);
 })();
+
+(function testReadableStreamData() {
+  var form = new FormData();
+
+  var util = require('util');
+  util.inherits(CustomReadable, Readable);
+
+  /**
+   * Custion readable constructor
+   * @param       {Object} opt options
+   * @constructor
+   */
+  function CustomReadable(opt) {
+    Readable.call(this, opt);
+    this._max = 2;
+    this._index = 1;
+  }
+
+  CustomReadable.prototype._read = function() {
+    var i = this._index++;
+    if (i > this._max) {
+      this.push(null);
+    } else {
+      this.push('' + i);
+    }
+  };
+  form.append('my_txt', new CustomReadable());
+
+  assert.throws(function() { form.getLengthSync(); }, /Cannot calculate proper length in synchronous way/);
+
+})();
diff --git a/test/integration/test-form-get-length.js b/test/integration/test-form-get-length.js
index 6b03069..6dabca7 100644
--- a/test/integration/test-form-get-length.js
+++ b/test/integration/test-form-get-length.js
@@ -3,6 +3,7 @@ var assert = common.assert;
 var FormData = require(common.dir.lib + '/form_data');
 var fake = require('fake').create();
 var fs = require('fs');
+var Readable = require('stream').Readable;
 
 (function testEmptyForm() {
   var form = new FormData();
@@ -89,3 +90,40 @@ var fs = require('fs');
   fake.expectAnytime(callback, [null, expectedLength]);
   form.getLength(callback);
 })();
+
+(function testReadableStreamData() {
+  var form = new FormData();
+  // var expectedLength = 0;
+
+  var util = require('util');
+  util.inherits(CustomReadable, Readable);
+
+  /**
+   * Custion readable constructor
+   * @param       {Object} opt options
+   * @constructor
+   */
+  function CustomReadable(opt) {
+    Readable.call(this, opt);
+    this._max = 2;
+    this._index = 1;
+  }
+
+  CustomReadable.prototype._read = function() {
+    var i = this._index++;
+    if (i > this._max) {
+      this.push(null);
+    } else {
+      this.push('' + i);
+    }
+  };
+  form.append('my_txt', new CustomReadable());
+
+  // expectedLength += form._overheadLength + form._lastBoundary().length;
+
+  // there is no way to determine the length of this readable stream.
+  var callback = fake.callback(arguments.callee.name + '-getLength');
+  fake.expectAnytime(callback, ['Unknown stream', undefined]);
+  form.getLength(function(err, len) { callback(err,len); });
+
+})();
diff --git a/test/integration/test-submit-readable-stream.js b/test/integration/test-submit-readable-stream.js
new file mode 100644
index 0000000..41896a4
--- /dev/null
+++ b/test/integration/test-submit-readable-stream.js
@@ -0,0 +1,54 @@
+var common = require('../common');
+var assert = common.assert;
+var http = require('http');
+var FormData = require(common.dir.lib + '/form_data');
+var Readable = require('stream').Readable;
+
+var server = http.createServer(function(req, res) {
+  assert.strictEqual(req.headers['Content-Length'], undefined);
+  res.writeHead(200);
+  res.end('done');
+});
+
+server.listen(common.port, function() {
+  var form = new FormData();
+
+  var util = require('util');
+  util.inherits(CustomReadable, Readable);
+
+  /**
+   * Custion readable constructor
+   * @param       {Object} opt options
+   * @constructor
+  */
+  function CustomReadable(opt) {
+    Readable.call(this, opt);
+    this._max = 2;
+    this._index = 1;
+  }
+
+  CustomReadable.prototype._read = function() {
+    var i = this._index++;
+    // console.error('send back read data');
+    if (i > this._max) {
+      this.push(null);
+    } else {
+      this.push('' + i);
+    }
+  };
+  form.append('readable', new CustomReadable());
+
+  form.submit('http://localhost:' + common.port + '/', function(err, res) {
+    if (err) {
+      throw err;
+    }
+
+    assert.strictEqual(res.statusCode, 200);
+
+    // unstuck new streams
+    res.resume();
+
+    server.close();
+  });
+
+});