diff --git a/packages/metro-bundler/src/JSTransformer/worker/minify.js b/packages/metro-bundler/src/JSTransformer/worker/minify.js
index deceb4cb..d6459ab4 100644
--- a/packages/metro-bundler/src/JSTransformer/worker/minify.js
+++ b/packages/metro-bundler/src/JSTransformer/worker/minify.js
@@ -15,17 +15,19 @@ const uglify = require('uglify-js');
 
 const {UGLIFY_JS_OUTPUT_OPTIONS} = require('./JsMinification');
 
-function minify(filename: string, code: string, sourceMap: ?string) {
-  const minifyResult = uglify.minify(code, {
+import type {MappingsMap} from '../../lib/SourceMap';
+
+function minify(filename: string, inputCode: string, sourceMap: ?MappingsMap) {
+  let {code, map} = uglify.minify(inputCode, {
     fromString: true,
     inSourceMap: sourceMap,
     outSourceMap: true,
     output: UGLIFY_JS_OUTPUT_OPTIONS,
   });
 
-  minifyResult.map = JSON.parse(minifyResult.map);
-  minifyResult.map.sources = [filename];
-  return minifyResult;
+  map = JSON.parse(map);
+  map.sources = [filename];
+  return {code, map};
 }
 
 module.exports = minify;
diff --git a/packages/metro-bundler/src/JSTransformer/worker/worker.js b/packages/metro-bundler/src/JSTransformer/worker/worker.js
index 011489a7..c674a1be 100644
--- a/packages/metro-bundler/src/JSTransformer/worker/worker.js
+++ b/packages/metro-bundler/src/JSTransformer/worker/worker.js
@@ -166,7 +166,7 @@ exports.transformAndExtractDependencies = (
 exports.minify = (
   filename: string,
   code: string,
-  sourceMap: string,
+  sourceMap: MappingsMap,
   callback: (error: ?Error, result: mixed) => mixed,
 ) => {
   var result;
diff --git a/packages/metro-bundler/src/ModuleGraph/types.flow.js b/packages/metro-bundler/src/ModuleGraph/types.flow.js
index 1352a927..8809cfbd 100644
--- a/packages/metro-bundler/src/ModuleGraph/types.flow.js
+++ b/packages/metro-bundler/src/ModuleGraph/types.flow.js
@@ -110,7 +110,7 @@ export type TransformResult = {|
   code: string,
   dependencies: Array<string>,
   dependencyMapName?: string,
-  map: ?Object,
+  map: ?MappingsMap,
 |};
 
 export type TransformResults = {[string]: TransformResult};
diff --git a/packages/metro-bundler/src/ModuleGraph/worker/optimize-module.js b/packages/metro-bundler/src/ModuleGraph/worker/optimize-module.js
index 89885163..4324d02a 100644
--- a/packages/metro-bundler/src/ModuleGraph/worker/optimize-module.js
+++ b/packages/metro-bundler/src/ModuleGraph/worker/optimize-module.js
@@ -19,6 +19,7 @@ const minify = require('../../JSTransformer/worker/minify');
 const sourceMap = require('source-map');
 
 import type {TransformedSourceFile, TransformResult} from '../types.flow';
+import type {MappingsMap, SourceMap} from '../../lib/SourceMap';
 
 export type OptimizationOptions = {|
   dev: boolean,
@@ -71,7 +72,7 @@ function optimize(transformed, file, originalCode, options): TransformResult {
     gen.code,
     inputMap && mergeSourceMaps(file, inputMap, gen.map),
   );
-  return {code: min.code, map: inputMap && min.map, dependencies};
+  return {code: min.code, map: min.map, dependencies};
 }
 
 function optimizeCode(code, map, filename, inliningOptions) {
@@ -86,7 +87,11 @@ function optimizeCode(code, map, filename, inliningOptions) {
   });
 }
 
-function mergeSourceMaps(file, originalMap, secondMap) {
+function mergeSourceMaps(
+  file: string,
+  originalMap: SourceMap,
+  secondMap: SourceMap,
+): MappingsMap {
   const merged = new sourceMap.SourceMapGenerator();
   const inputMap = new sourceMap.SourceMapConsumer(originalMap);
   new sourceMap.SourceMapConsumer(secondMap)