diff --git a/packages/compiler-cli/src/ngtsc/program.ts b/packages/compiler-cli/src/ngtsc/program.ts
index 57e0e706fe..4c53cc12f2 100644
--- a/packages/compiler-cli/src/ngtsc/program.ts
+++ b/packages/compiler-cli/src/ngtsc/program.ts
@@ -116,7 +116,7 @@ export class NgtscProgram implements api.Program {
     rootFiles.push(this.typeCheckFilePath);
 
     let entryPoint: AbsoluteFsPath|null = null;
-    if (options.flatModuleOutFile !== undefined) {
+    if (options.flatModuleOutFile != null && options.flatModuleOutFile !== '') {
       entryPoint = findFlatIndexEntryPoint(normalizedRootNames);
       if (entryPoint === null) {
         // This error message talks specifically about having a single .ts file in "files". However
diff --git a/packages/compiler-cli/test/ngtsc/env.ts b/packages/compiler-cli/test/ngtsc/env.ts
index 445c70e569..cf274ab618 100644
--- a/packages/compiler-cli/test/ngtsc/env.ts
+++ b/packages/compiler-cli/test/ngtsc/env.ts
@@ -145,7 +145,8 @@ export class NgtscTestEnvironment {
     this.multiCompileHostExt.invalidate(absFilePath);
   }
 
-  tsconfig(extraOpts: {[key: string]: string | boolean} = {}, extraRootDirs?: string[]): void {
+  tsconfig(extraOpts: {[key: string]: string | boolean | null} = {}, extraRootDirs?: string[]):
+      void {
     const tsconfig: {[key: string]: any} = {
       extends: './tsconfig-base.json',
       angularCompilerOptions: {...extraOpts, enableIvy: true},
diff --git a/packages/compiler-cli/test/ngtsc/ngtsc_spec.ts b/packages/compiler-cli/test/ngtsc/ngtsc_spec.ts
index 018353e709..d1daf105f5 100644
--- a/packages/compiler-cli/test/ngtsc/ngtsc_spec.ts
+++ b/packages/compiler-cli/test/ngtsc/ngtsc_spec.ts
@@ -2967,6 +2967,35 @@ runInEachFileSystem(os => {
         expect(env.getContents('./public-api/index.js')).toContain(`export * from '../test';`);
       });
 
+      it('should not throw if "flatModuleOutFile" is set to null', () => {
+        env.tsconfig({
+          'flatModuleOutFile': null,
+        });
+
+        env.write('test.ts', `export const SOME_EXPORT = 'some-export'`);
+        // The "driveMain" method automatically ensures that there is no
+        // exception and that the build succeeded.
+        env.driveMain();
+      });
+
+      it('should not throw or produce flat module index if "flatModuleOutFile" is set to ' +
+             'empty string',
+         () => {
+           env.tsconfig({
+             'flatModuleOutFile': '',
+           });
+
+           env.write('test.ts', `export const SOME_EXPORT = 'some-export'`);
+           // The "driveMain" method automatically ensures that there is no
+           // exception and that the build succeeded.
+           env.driveMain();
+           // Previously ngtsc incorrectly tried generating a flat module index
+           // file if the "flatModuleOutFile" was set to an empty string. ngtsc
+           // just wrote the bundle file with an empty filename (just extension).
+           env.assertDoesNotExist('.js');
+           env.assertDoesNotExist('.d.ts');
+         });
+
       it('should report an error when a flat module index is requested but no entrypoint can be determined',
          () => {
            env.tsconfig({'flatModuleOutFile': 'flat.js'});