diff --git a/packages/metro-bundler/src/ModuleGraph/output/__tests__/indexed-ram-bundle-test.js b/packages/metro-bundler/src/ModuleGraph/output/__tests__/indexed-ram-bundle-test.js
index 64f6b6ad..ab768c23 100644
--- a/packages/metro-bundler/src/ModuleGraph/output/__tests__/indexed-ram-bundle-test.js
+++ b/packages/metro-bundler/src/ModuleGraph/output/__tests__/indexed-ram-bundle-test.js
@@ -34,7 +34,7 @@ const idsForPath = ({path}) => {
 
 beforeAll(() => {
   modules = [
-    makeModule('a', [], 'script'),
+    makeModule('a', []),
     makeModule('b', ['c']),
     makeModule('c', ['f']),
     makeModule('d', ['e']),
diff --git a/packages/metro-bundler/src/ModuleGraph/output/__tests__/multiple-files-ram-bundle-test.js b/packages/metro-bundler/src/ModuleGraph/output/__tests__/multiple-files-ram-bundle-test.js
index db008437..8c78bf5d 100644
--- a/packages/metro-bundler/src/ModuleGraph/output/__tests__/multiple-files-ram-bundle-test.js
+++ b/packages/metro-bundler/src/ModuleGraph/output/__tests__/multiple-files-ram-bundle-test.js
@@ -35,7 +35,7 @@ const idsForPath = ({path}) => {
 
 beforeAll(() => {
   modules = [
-    makeModule('a', [], 'script'),
+    makeModule('a', []),
     makeModule('b'),
     makeModule('c', ['f']),
     makeModule('d', ['e']),
diff --git a/packages/metro-bundler/src/ModuleGraph/output/indexed-ram-bundle.js b/packages/metro-bundler/src/ModuleGraph/output/indexed-ram-bundle.js
index e2b3b8a3..bcf54b38 100644
--- a/packages/metro-bundler/src/ModuleGraph/output/indexed-ram-bundle.js
+++ b/packages/metro-bundler/src/ModuleGraph/output/indexed-ram-bundle.js
@@ -13,6 +13,7 @@
 'use strict';
 
 const buildSourceMapWithMetaData = require('../../shared/output/unbundle/build-unbundle-sourcemap-with-metadata.js');
+const invariant = require('invariant');
 
 const {createRamBundleGroups} = require('../../Bundler/util');
 const {
@@ -36,6 +37,12 @@ function asIndexedRamBundle({
   const [startup, deferred] = partition(modules, preloadedModules);
   const startupModules = Array.from(concat(startup, requireCalls));
   const deferredModules = deferred.map(m => toModuleTransport(m, idsForPath));
+  for (const m of deferredModules) {
+    invariant(
+      m.id >= 0,
+      'A script (non-module) cannot be part of the deferred modules of a RAM bundle',
+    );
+  }
   const ramGroups = createRamBundleGroups(
     ramGroupHeads || [],
     deferredModules,
diff --git a/packages/metro-bundler/src/ModuleGraph/output/util.js b/packages/metro-bundler/src/ModuleGraph/output/util.js
index d5e30648..ca2aa8cd 100644
--- a/packages/metro-bundler/src/ModuleGraph/output/util.js
+++ b/packages/metro-bundler/src/ModuleGraph/output/util.js
@@ -117,7 +117,8 @@ exports.toModuleTransport = (module: Module, idsForPath: IdsForPathFn) => {
   return {
     code: getModuleCode(module, x => idsForPath(x).moduleId),
     dependencies,
-    id: idsForPath(file).localId,
+    // ID is required but we provide an invalid one for "script"s.
+    id: file.type === 'module' ? idsForPath(file).localId : -1,
     map: file.map,
     name: file.path,
     sourcePath: file.path,