diff --git a/flow-typed/babel.js.flow b/flow-typed/babel.js.flow
index 5893a444..3fc6aa60 100644
--- a/flow-typed/babel.js.flow
+++ b/flow-typed/babel.js.flow
@@ -98,6 +98,12 @@ type TransformResult = {
   ignored: boolean,
   map: ?_SourceMap,
 };
+// https://github.com/babel/babel/blob/master/packages/babel-generator/src/buffer.js#L42
+type GeneratorResult = {
+  code: string,
+  map: ?_SourceMap,
+  rawMappings: ?Array<RawMapping>,
+};
 type VisitFn = <State>(path: Object, state: State) => any;
 
 declare module 'babel-core' {
@@ -132,10 +138,12 @@ type RawMapping = {
   source?: string,
 };
 
+// https://github.com/babel/babel/tree/master/packages/babel-generator
 declare module 'babel-generator' {
   declare type RawMapping = RawMapping;
-  declare module.exports: (
+  declare module.exports: {default: (
     ast: Ast,
     options?: GeneratorOptions,
-  ) => TransformResult & {rawMappings: ?Array<RawMapping>};
+    code?: string | {[string]: string},
+  ) => GeneratorResult};
 }
diff --git a/packages/metro/src/ModuleGraph/worker/generate.js b/packages/metro/src/ModuleGraph/worker/generate.js
index d1ebdf79..966b62b8 100644
--- a/packages/metro/src/ModuleGraph/worker/generate.js
+++ b/packages/metro/src/ModuleGraph/worker/generate.js
@@ -35,7 +35,9 @@ function generate(
     sourceCode,
   );
 
-  delete generated.map.sourcesContent;
+  if (generated.map) {
+    delete generated.map.sourcesContent;
+  }
   return generated;
 }
 
diff --git a/packages/metro/src/ModuleGraph/worker/optimize-module.js b/packages/metro/src/ModuleGraph/worker/optimize-module.js
index ebbdff94..a4064ce1 100644
--- a/packages/metro/src/ModuleGraph/worker/optimize-module.js
+++ b/packages/metro/src/ModuleGraph/worker/optimize-module.js
@@ -90,7 +90,7 @@ function optimize(transformed: TransformResult, file, options) {
 
   const min = minify.withSourceMap(
     gen.code,
-    inputMap && mergeSourceMaps(file, inputMap, gen.map),
+    inputMap && gen.map && mergeSourceMaps(file, inputMap, gen.map),
     file,
   );
   return {code: min.code, map: min.map, dependencies};