diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 986546772e..6001760c4a 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -49,7 +49,7 @@ namespace ts {
 
         const compilerOptions = host.getCompilerOptions();
         const languageVersion = compilerOptions.target || ScriptTarget.ES3;
-        const modulekind = compilerOptions.module ? compilerOptions.module : languageVersion === ScriptTarget.ES6 ? ModuleKind.ES6 : ModuleKind.CommonJS;
+        const modulekind = typeof compilerOptions.module === "number" ? compilerOptions.module : languageVersion === ScriptTarget.ES6 ? ModuleKind.ES6 : ModuleKind.CommonJS;
         const allowSyntheticDefaultImports = typeof compilerOptions.allowSyntheticDefaultImports !== "undefined" ? compilerOptions.allowSyntheticDefaultImports : modulekind === ModuleKind.System;
 
         const emitResolver = createResolver();
diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index 447fdd9799..1a8a53d517 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/src/compiler/diagnosticMessages.json
@@ -439,6 +439,10 @@
         "category": "Error",
         "code": 1147
     },
+    "Cannot compile modules unless the '--module' flag is provided. Consider setting the 'module' compiler option in a 'tsconfig.json' file.": {
+        "category": "Error",
+        "code": 1148
+    },
     "File name '{0}' differs from already included file name '{1}' only in casing": {
         "category": "Error",
         "code": 1149
@@ -2127,6 +2131,10 @@
         "category": "Error",
         "code": 5042
     },
+    "Option 'isolatedModules' can only be used when either option '--module' is provided or option 'target' is 'ES2015' or higher.": {
+        "category": "Error",
+        "code": 5047
+    },
     "Option 'inlineSources' can only be used when either option '--inlineSourceMap' or option '--sourceMap' is provided.": {
         "category": "Error",
         "code": 5051
@@ -2163,7 +2171,7 @@
         "category": "Error",
         "code": 5059
     },
-
+
 
     "Concatenate and emit output to single file.": {
         "category": "Message",
diff --git a/src/compiler/program.ts b/src/compiler/program.ts
index 47b6d44ccb..3c75f31add 100644
--- a/src/compiler/program.ts
+++ b/src/compiler/program.ts
@@ -1288,13 +1288,23 @@ namespace ts {
             const languageVersion = options.target || ScriptTarget.ES3;
             const outFile = options.outFile || options.out;
 
+            const firstExternalModuleSourceFile = forEach(files, f => isExternalModule(f) ? f : undefined);
             if (options.isolatedModules) {
+                if (!options.module && languageVersion < ScriptTarget.ES6) {
+                    programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Option_isolatedModules_can_only_be_used_when_either_option_module_is_provided_or_option_target_is_ES2015_or_higher));
+                }
+
                 const firstNonExternalModuleSourceFile = forEach(files, f => !isExternalModule(f) && !isDeclarationFile(f) ? f : undefined);
                 if (firstNonExternalModuleSourceFile) {
                     const span = getErrorSpanForNode(firstNonExternalModuleSourceFile, firstNonExternalModuleSourceFile);
                     programDiagnostics.add(createFileDiagnostic(firstNonExternalModuleSourceFile, span.start, span.length, Diagnostics.Cannot_compile_namespaces_when_the_isolatedModules_flag_is_provided));
                 }
             }
+            else if (firstExternalModuleSourceFile && languageVersion < ScriptTarget.ES6 && !options.module) {
+                // We cannot use createDiagnosticFromNode because nodes do not have parents yet
+                const span = getErrorSpanForNode(firstExternalModuleSourceFile, firstExternalModuleSourceFile.externalModuleIndicator);
+                programDiagnostics.add(createFileDiagnostic(firstExternalModuleSourceFile, span.start, span.length, Diagnostics.Cannot_compile_modules_unless_the_module_flag_is_provided_Consider_setting_the_module_compiler_option_in_a_tsconfig_json_file));
+            }
 
             // Cannot specify module gen target of es6 when below es6
             if (options.module === ModuleKind.ES6 && languageVersion < ScriptTarget.ES6) {
diff --git a/src/compiler/utilities.ts b/src/compiler/utilities.ts
index 3406f40bf3..193807af1a 100644
--- a/src/compiler/utilities.ts
+++ b/src/compiler/utilities.ts
@@ -2011,7 +2011,7 @@ namespace ts {
     }
 
     export function getEmitModuleKind(compilerOptions: CompilerOptions) {
-        return compilerOptions.module ?
+        return typeof compilerOptions.module === "number" ?
             compilerOptions.module :
             getEmitScriptTarget(compilerOptions) === ScriptTarget.ES6 ? ModuleKind.ES6 : ModuleKind.CommonJS;
     }