Uncaught Error: Unexpected value 'undefined' imported by the module 'AppModule'

Fiz exatamente como na aula e ao rodar a aplicação, retornou este erro no browser:

Uncaught Error: Unexpected value 'undefined' imported by the module 'AppModule'
    at syntaxError (compiler.js:1021)
    at compiler.js:10589
    at Array.forEach (<anonymous>)
    at CompileMetadataResolver.push../node_modules/@angular/compiler/fesm5/compiler.js.CompileMetadataResolver.getNgModuleMetadata (compiler.js:10558)
    at JitCompiler.push../node_modules/@angular/compiler/fesm5/compiler.js.JitCompiler._loadModules (compiler.js:23876)
    at JitCompiler.push../node_modules/@angular/compiler/fesm5/compiler.js.JitCompiler._compileModuleAndComponents (compiler.js:23857)
    at JitCompiler.push../node_modules/@angular/compiler/fesm5/compiler.js.JitCompiler.compileModuleAsync (compiler.js:23817)
    at CompilerImpl.push../node_modules/@angular/platform-browser-dynamic/fesm5/platform-browser-dynamic.js.CompilerImpl.compileModuleAsync (platform-browser-dynamic.js:143)
    at PlatformRef.push../node_modules/@angular/core/fesm5/core.js.PlatformRef.bootstrapModule (core.js:4344)
    at Object../src/main.ts (main.ts:11)