diff --git a/packages/metro-bundler/src/Bundler/Bundle.js b/packages/metro-bundler/src/Bundler/Bundle.js
index e9a58550..a53d2b18 100644
--- a/packages/metro-bundler/src/Bundler/Bundle.js
+++ b/packages/metro-bundler/src/Bundler/Bundle.js
@@ -49,14 +49,14 @@ class Bundle extends BundleBase {
   _sourceMap: string | null;
   _sourceMapFormat: SourceMapFormat;
   _sourceMapUrl: ?string;
-  postProcessBundleSourcemap: ?PostProcessBundleSourcemap;
+  postProcessBundleSourcemap: PostProcessBundleSourcemap;
 
   constructor({sourceMapUrl, dev, minify, ramGroups, postProcessBundleSourcemap}: {
     sourceMapUrl: ?string,
     dev?: boolean,
     minify?: boolean,
     ramGroups?: Array<string>,
-    postProcessBundleSourcemap?: PostProcessBundleSourcemap,
+    postProcessBundleSourcemap: PostProcessBundleSourcemap,
   } = {}) {
     super();
     this._sourceMap = null;
diff --git a/packages/metro-bundler/src/Bundler/index.js b/packages/metro-bundler/src/Bundler/index.js
index efd0baaa..612e973f 100644
--- a/packages/metro-bundler/src/Bundler/index.js
+++ b/packages/metro-bundler/src/Bundler/index.js
@@ -41,7 +41,7 @@ const VERSION = require('../../package.json').version;
 import type AssetServer from '../AssetServer';
 import type Module, {HasteImpl} from '../node-haste/Module';
 import type ResolutionResponse from '../node-haste/DependencyGraph/ResolutionResponse';
-import type {MappingsMap} from '../lib/SourceMap';
+import type {MappingsMap, SourceMap} from '../lib/SourceMap';
 import type {Options as JSTransformerOptions} from '../JSTransformer/worker';
 import type {Reporter} from '../lib/reporting';
 import type {TransformCache} from '../lib/TransformCaching';
@@ -113,10 +113,10 @@ export type PostMinifyProcess = ({
 }) => {code: string, map: MappingsMap};
 
 export type PostProcessBundleSourcemap = ({
-  code: string,
-  map: string,
+  code: Buffer | string,
+  map: SourceMap,
   outFileName: string,
-}) => {code: string, map: string};
+}) => {code: Buffer | string, map: SourceMap | string};
 
 type Options = {|
   +allowBundleUpdates: boolean,
@@ -132,7 +132,7 @@ type Options = {|
   +platforms: Array<string>,
   +polyfillModuleNames: Array<string>,
   +postMinifyProcess: PostMinifyProcess,
-  +postProcessBundleSourcemap?: PostProcessBundleSourcemap,
+  +postProcessBundleSourcemap: PostProcessBundleSourcemap,
   +postProcessModules?: PostProcessModules,
   +projectRoots: $ReadOnlyArray<string>,
   +providesModuleNodeModules?: Array<string>,
diff --git a/packages/metro-bundler/src/shared/output/bundle.js b/packages/metro-bundler/src/shared/output/bundle.js
index 81ac4c1b..ed72c1a8 100644
--- a/packages/metro-bundler/src/shared/output/bundle.js
+++ b/packages/metro-bundler/src/shared/output/bundle.js
@@ -29,14 +29,18 @@ function buildBundle(packagerClient: Server, requestOptions: RequestOptions) {
   });
 }
 
-function createCodeWithMap(bundle: Bundle, dev: boolean, sourceMapSourcesRoot?: string): * {
+function createCodeWithMap(
+  bundle: Bundle,
+  dev: boolean,
+  sourceMapSourcesRoot?: string,
+): {code: string, map: SourceMap} {
   const map = bundle.getSourceMap({dev});
   const sourceMap = relativizeSourceMap(
     typeof map === 'string' ? (JSON.parse(map): SourceMap) : map,
     sourceMapSourcesRoot);
   return {
     code: bundle.getSource({dev}),
-    map: JSON.stringify(sourceMap),
+    map: sourceMap,
   };
 }
 
@@ -55,9 +59,10 @@ function saveBundleAndMap(
 
   log('start');
   const origCodeWithMap = createCodeWithMap(bundle, !!dev, sourcemapSourcesRoot);
-  const codeWithMap = bundle.postProcessBundleSourcemap ?
-    bundle.postProcessBundleSourcemap({...origCodeWithMap, outFileName: bundleOutput}) :
-    origCodeWithMap;
+  const codeWithMap = bundle.postProcessBundleSourcemap({
+    ...origCodeWithMap,
+    outFileName: bundleOutput,
+  });
   log('finish');
 
   log('Writing bundle output to:', bundleOutput);
@@ -73,7 +78,10 @@ function saveBundleAndMap(
 
   if (sourcemapOutput) {
     log('Writing sourcemap output to:', sourcemapOutput);
-    const writeMap = writeFile(sourcemapOutput, codeWithMap.map, null);
+    const map = typeof codeWithMap.map !== 'string'
+      ? JSON.stringify(codeWithMap.map)
+      : codeWithMap.map;
+    const writeMap = writeFile(sourcemapOutput, map, null);
     writeMap.then(() => log('Done writing sourcemap output'));
     return Promise.all([writeBundle, writeMetadata, writeMap]);
   } else {
diff --git a/packages/metro-bundler/src/shared/output/meta.js b/packages/metro-bundler/src/shared/output/meta.js
index 4c110ada..b636b423 100644
--- a/packages/metro-bundler/src/shared/output/meta.js
+++ b/packages/metro-bundler/src/shared/output/meta.js
@@ -22,25 +22,33 @@ const constantFor = encoding =>
   /^(?:utf-?16(?:le)?|ucs-?2)$/.test(encoding) ? 3 : 0;
 
 module.exports = function(
-  code: string,
+  code: Buffer | string,
   encoding: 'ascii' | 'utf8' | 'utf16le' = 'utf8',
 ): Buffer {
+  const buffer: Buffer = asBuffer(code, encoding);
   const hash = crypto.createHash('sha1');
-  // remove `new Buffer` calls when RN drops support for Node 4
-  hash.update(Buffer.from ? Buffer.from(code, encoding) : new Buffer(code, encoding));
+  hash.update(buffer);
   const digest = hash.digest();
   const signature = Buffer.alloc ? Buffer.alloc(digest.length + 1) : new Buffer(digest.length + 1);
   digest.copy(signature);
   signature.writeUInt8(
-    constantFor(tryAsciiPromotion(code, encoding)),
+    constantFor(tryAsciiPromotion(buffer, encoding)),
     signature.length - 1);
   return signature;
 };
 
-function tryAsciiPromotion(string, encoding) {
+function tryAsciiPromotion(buffer, encoding) {
   if (!isUTF8(encoding)) { return encoding; }
-  for (let i = 0, n = string.length; i < n; i++) {
-    if (string.charCodeAt(i) > 0x7f) { return encoding; }
+  for (let i = 0, n = buffer.length; i < n; i++) {
+    if (buffer[i] > 0x7f) { return encoding; }
   }
   return 'ascii';
 }
+
+function asBuffer(x, encoding): Buffer {
+  if (typeof x !== 'string') {
+    return x;
+  }
+  // remove `new Buffer` calls when RN drops support for Node 4
+  return Buffer.from ? Buffer.from(x, encoding) : new Buffer(x, encoding);
+}