diff --git a/CHANGELOG.md b/CHANGELOG.md
index 119227f..c87e445 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,6 @@
 ## 0.12.0+4-dev
 
-* Internal changes.
+* Fix a bug setting the `'content-type'` header in `MultipartRequest`.
 
 ## 0.12.0+3
 
diff --git a/lib/src/multipart_request.dart b/lib/src/multipart_request.dart
index 699c643..4d89c6c 100644
--- a/lib/src/multipart_request.dart
+++ b/lib/src/multipart_request.dart
@@ -85,13 +85,15 @@
   /// Freezes all mutable fields and returns a single-subscription [ByteStream]
   /// that will emit the request body.
   @override
-  ByteStream finalize() => ByteStream(_finalize());
-
-  Stream<List<int>> _finalize() async* {
-    // TODO(nweiz): freeze fields and files
-    var boundary = _boundaryString();
+  ByteStream finalize() {
+    // TODO: freeze fields and files
+    final boundary = _boundaryString();
     headers['content-type'] = 'multipart/form-data; boundary=$boundary';
     super.finalize();
+    return ByteStream(_finalize(boundary));
+  }
+
+  Stream<List<int>> _finalize(String boundary) async* {
     const line = [13, 10]; // \r\n
     final separator = utf8.encode('--$boundary\r\n');
     final close = utf8.encode('--$boundary--\r\n');
diff --git a/test/io/client_test.dart b/test/io/client_test.dart
index f8c83f7..25bbd6b 100644
--- a/test/io/client_test.dart
+++ b/test/io/client_test.dart
@@ -4,6 +4,7 @@
 
 @TestOn('vm')
 
+import 'dart:convert';
 import 'dart:io';
 
 import 'package:http/http.dart' as http;
@@ -106,4 +107,18 @@
     request.sink.add('{"hello": "world"}'.codeUnits);
     request.sink.close();
   });
+
+  test('sends a MultipartRequest with correct content-type header', () async {
+    var client = http.Client();
+    var request = http.MultipartRequest('POST', serverUrl);
+
+    var response = await client.send(request);
+
+    var bytesString = await response.stream.bytesToString();
+    client.close();
+
+    var headers = jsonDecode(bytesString)['headers'] as Map<String, dynamic>;
+    var contentType = (headers['content-type'] as List).single;
+    expect(contentType, startsWith('multipart/form-data; boundary='));
+  });
 }