diff --git a/packages/metro-bundler/react-packager.js b/packages/metro-bundler/react-packager.js
index 24d001c1..457a877f 100644
--- a/packages/metro-bundler/react-packager.js
+++ b/packages/metro-bundler/react-packager.js
@@ -34,6 +34,7 @@ type Options = {
   projectRoots: Array<string>,
   reporter?: Reporter,
   +sourceExts: ?Array<string>,
+  +transformModulePath: string,
   watch?: boolean,
 };
 
diff --git a/packages/metro-bundler/src/Bundler/__tests__/Bundler-test.js b/packages/metro-bundler/src/Bundler/__tests__/Bundler-test.js
index 5a171a80..bfcfa787 100644
--- a/packages/metro-bundler/src/Bundler/__tests__/Bundler-test.js
+++ b/packages/metro-bundler/src/Bundler/__tests__/Bundler-test.js
@@ -25,6 +25,7 @@ jest
   .mock('../Bundle')
   .mock('../HMRBundle')
   .mock('../../Logger')
+  .mock('/path/to/transformer.js', () => ({}), {virtual: true})
   ;
 
 var Bundler = require('../');
@@ -36,6 +37,7 @@ const os = require('os');
 
 const {any, objectContaining} = expect;
 
+
 var commonOptions = {
   allowBundleUpdates: false,
   assetExts: defaults.assetExts,
@@ -44,6 +46,7 @@ var commonOptions = {
   platforms: defaults.platforms,
   resetCache: false,
   sourceExts: defaults.sourceExts,
+  transformModulePath: '/path/to/transformer.js',
   watch: false,
 };
 
@@ -95,6 +98,10 @@ describe('Bundler', function() {
     });
     Resolver.load = jest.fn().mockImplementation(opts => Promise.resolve(new Resolver(opts)));
 
+    fs.__setMockFilesystem({
+      'path': {'to': {'transformer.js': ''}},
+    });
+
     fs.statSync.mockImplementation(function() {
       return {
         isDirectory: () => true,
diff --git a/packages/metro-bundler/src/Bundler/index.js b/packages/metro-bundler/src/Bundler/index.js
index d64820e9..bf88f784 100644
--- a/packages/metro-bundler/src/Bundler/index.js
+++ b/packages/metro-bundler/src/Bundler/index.js
@@ -133,7 +133,7 @@ type Options = {|
   +reporter: Reporter,
   +resetCache: boolean,
   +sourceExts: Array<string>,
-  +transformModulePath?: string,
+  +transformModulePath: string,
   +transformTimeoutInterval: ?number,
   +watch: boolean,
 |};
@@ -155,15 +155,9 @@ class Bundler {
 
     opts.projectRoots.forEach(verifyRootExists);
 
-    let transformModuleHash;
-    try {
-      /* $FlowFixMe: if transformModulePath is null it'll just be caught */
-      const transformModuleStr = fs.readFileSync(opts.transformModulePath);
-      transformModuleHash =
-        crypto.createHash('sha1').update(transformModuleStr).digest('hex');
-    } catch (error) {
-      transformModuleHash = '';
-    }
+    const transformModuleStr = fs.readFileSync(opts.transformModulePath);
+    const transformModuleHash =
+      crypto.createHash('sha1').update(transformModuleStr).digest('hex');
 
     const stableProjectRoots = opts.projectRoots.map(p => {
       return path.relative(path.join(__dirname, '../../../..'), p);
@@ -197,7 +191,6 @@ class Bundler {
     const maxWorkerCount = Bundler.getMaxWorkerCount();
 
     this._transformer = new Transformer(
-      /* $FlowFixMe: in practice it's always here. */
       opts.transformModulePath,
       maxWorkerCount,
       {
diff --git a/packages/metro-bundler/src/Server/index.js b/packages/metro-bundler/src/Server/index.js
index 9fb3fe80..0e0f5371 100644
--- a/packages/metro-bundler/src/Server/index.js
+++ b/packages/metro-bundler/src/Server/index.js
@@ -75,7 +75,7 @@ type Options = {
   resetCache?: boolean,
   silent?: boolean,
   +sourceExts: ?Array<string>,
-  transformModulePath?: string,
+  +transformModulePath: string,
   transformTimeoutInterval?: number,
   watch?: boolean,
 };
@@ -131,7 +131,7 @@ class Server {
     resetCache: boolean,
     silent: boolean,
     +sourceExts: Array<string>,
-    transformModulePath: void | string,
+    +transformModulePath: string,
     transformTimeoutInterval: ?number,
     watch: boolean,
   };