diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index 899b4292f04..710981fc196 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/src/compiler/diagnosticMessages.json
@@ -2751,7 +2751,12 @@
     "Resolving real path for '{0}', result '{1}'": {
         "category": "Message",
         "code": 6130
-    },    
+    },
+    "Cannot compile modules using option '{0}' unless the '--module' flag is provided with a valid module type.": {
+        "category": "Error",
+        "code": 6131
+    },
+
     "Variable '{0}' implicitly has an '{1}' type.": {
         "category": "Error",
         "code": 7005
diff --git a/src/compiler/program.ts b/src/compiler/program.ts
index 52547cb165d..4f9b82c9c6d 100644
--- a/src/compiler/program.ts
+++ b/src/compiler/program.ts
@@ -2089,8 +2089,14 @@ namespace ts {
             }
 
             // Cannot specify module gen that isn't amd or system with --out
-            if (outFile && options.module && !(options.module === ModuleKind.AMD || options.module === ModuleKind.System)) {
-                programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Only_amd_and_system_modules_are_supported_alongside_0, options.out ? "out" : "outFile"));
+            if (outFile) {
+                if (options.module && !(options.module === ModuleKind.AMD || options.module === ModuleKind.System)) {
+                    programDiagnostics.add(createCompilerDiagnostic(Diagnostics.Only_amd_and_system_modules_are_supported_alongside_0, options.out ? "out" : "outFile"));
+                }
+                else if (options.module === undefined && firstExternalModuleSourceFile) {
+                    const span = getErrorSpanForNode(firstExternalModuleSourceFile, firstExternalModuleSourceFile.externalModuleIndicator);
+                    programDiagnostics.add(createFileDiagnostic(firstExternalModuleSourceFile, span.start, span.length, Diagnostics.Cannot_compile_modules_using_option_0_unless_the_module_flag_is_provided_with_a_valid_module_type, options.out ? "out" : "outFile"));
+                }
             }
 
             // there has to be common source directory if user specified --outdir || --sourceRoot
diff --git a/tests/baselines/reference/outModuleConcatUnspecifiedModuleKind.errors.txt b/tests/baselines/reference/outModuleConcatUnspecifiedModuleKind.errors.txt
new file mode 100644
index 00000000000..f0613367140
--- /dev/null
+++ b/tests/baselines/reference/outModuleConcatUnspecifiedModuleKind.errors.txt
@@ -0,0 +1,11 @@
+tests/cases/compiler/a.ts(2,14): error TS6131: Cannot compile modules using option 'outFile' unless the '--module' flag is provided with a valid module type.
+
+
+==== tests/cases/compiler/a.ts (1 errors) ====
+    
+    export class A { } // module
+                 ~
+!!! error TS6131: Cannot compile modules using option 'outFile' unless the '--module' flag is provided with a valid module type.
+    
+==== tests/cases/compiler/b.ts (0 errors) ====
+    var x = 0; // global
\ No newline at end of file
diff --git a/tests/baselines/reference/outModuleConcatUnspecifiedModuleKind.js b/tests/baselines/reference/outModuleConcatUnspecifiedModuleKind.js
new file mode 100644
index 00000000000..48548400f7f
--- /dev/null
+++ b/tests/baselines/reference/outModuleConcatUnspecifiedModuleKind.js
@@ -0,0 +1,11 @@
+//// [tests/cases/compiler/outModuleConcatUnspecifiedModuleKind.ts] ////
+
+//// [a.ts]
+
+export class A { } // module
+
+//// [b.ts]
+var x = 0; // global
+
+//// [out.js]
+var x = 0; // global
diff --git a/tests/baselines/reference/typeReferenceDirectives11.errors.txt b/tests/baselines/reference/typeReferenceDirectives11.errors.txt
new file mode 100644
index 00000000000..b19efcebb90
--- /dev/null
+++ b/tests/baselines/reference/typeReferenceDirectives11.errors.txt
@@ -0,0 +1,18 @@
+/mod1.ts(2,17): error TS6131: Cannot compile modules using option 'out' unless the '--module' flag is provided with a valid module type.
+
+
+==== /mod2.ts (0 errors) ====
+    
+    import {foo} from "./mod1";
+    export const bar = foo();
+==== /types/lib/index.d.ts (0 errors) ====
+    
+    
+    interface Lib { x }
+    
+==== /mod1.ts (1 errors) ====
+    
+    export function foo(): Lib { return {x: 1} }
+                    ~~~
+!!! error TS6131: Cannot compile modules using option 'out' unless the '--module' flag is provided with a valid module type.
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/typeReferenceDirectives11.symbols b/tests/baselines/reference/typeReferenceDirectives11.symbols
deleted file mode 100644
index 875bb2a0900..00000000000
--- a/tests/baselines/reference/typeReferenceDirectives11.symbols
+++ /dev/null
@@ -1,23 +0,0 @@
-=== /mod2.ts ===
-
-import {foo} from "./mod1";
->foo : Symbol(foo, Decl(mod2.ts, 1, 8))
-
-export const bar = foo();
->bar : Symbol(bar, Decl(mod2.ts, 2, 12))
->foo : Symbol(foo, Decl(mod2.ts, 1, 8))
-
-=== /types/lib/index.d.ts ===
-
-
-interface Lib { x }
->Lib : Symbol(Lib, Decl(index.d.ts, 0, 0))
->x : Symbol(Lib.x, Decl(index.d.ts, 2, 15))
-
-=== /mod1.ts ===
-
-export function foo(): Lib { return {x: 1} }
->foo : Symbol(foo, Decl(mod1.ts, 0, 0))
->Lib : Symbol(Lib, Decl(index.d.ts, 0, 0))
->x : Symbol(x, Decl(mod1.ts, 1, 37))
-
diff --git a/tests/baselines/reference/typeReferenceDirectives11.types b/tests/baselines/reference/typeReferenceDirectives11.types
deleted file mode 100644
index 93d80b7988a..00000000000
--- a/tests/baselines/reference/typeReferenceDirectives11.types
+++ /dev/null
@@ -1,26 +0,0 @@
-=== /mod2.ts ===
-
-import {foo} from "./mod1";
->foo : () => Lib
-
-export const bar = foo();
->bar : Lib
->foo() : Lib
->foo : () => Lib
-
-=== /types/lib/index.d.ts ===
-
-
-interface Lib { x }
->Lib : Lib
->x : any
-
-=== /mod1.ts ===
-
-export function foo(): Lib { return {x: 1} }
->foo : () => Lib
->Lib : Lib
->{x: 1} : { x: number; }
->x : number
->1 : number
-
diff --git a/tests/baselines/reference/typeReferenceDirectives12.errors.txt b/tests/baselines/reference/typeReferenceDirectives12.errors.txt
new file mode 100644
index 00000000000..8eee7c04ee3
--- /dev/null
+++ b/tests/baselines/reference/typeReferenceDirectives12.errors.txt
@@ -0,0 +1,37 @@
+/main.ts(1,14): error TS6131: Cannot compile modules using option 'out' unless the '--module' flag is provided with a valid module type.
+
+
+==== /mod2.ts (0 errors) ====
+    import { Cls } from "./main";
+    import "./mod1";
+    
+    export const cls = Cls;
+    export const foo = new Cls().foo();
+    export const bar = Cls.bar();
+==== /types/lib/index.d.ts (0 errors) ====
+    
+    
+    interface Lib { x }
+    
+==== /main.ts (1 errors) ====
+    export class Cls {
+                 ~~~
+!!! error TS6131: Cannot compile modules using option 'out' unless the '--module' flag is provided with a valid module type.
+        x
+    }
+    
+==== /mod1.ts (0 errors) ====
+    /// <reference types="lib" />
+    
+    import {Cls} from "./main";
+    Cls.prototype.foo = function() { return undefined; }
+    
+    declare module "./main" {
+        interface Cls {
+            foo(): Lib;
+        }
+        namespace Cls {
+            function bar(): Lib;
+        }
+    }
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/typeReferenceDirectives12.symbols b/tests/baselines/reference/typeReferenceDirectives12.symbols
deleted file mode 100644
index 30b24c39284..00000000000
--- a/tests/baselines/reference/typeReferenceDirectives12.symbols
+++ /dev/null
@@ -1,68 +0,0 @@
-=== /mod2.ts ===
-import { Cls } from "./main";
->Cls : Symbol(Cls, Decl(mod2.ts, 0, 8))
-
-import "./mod1";
-
-export const cls = Cls;
->cls : Symbol(cls, Decl(mod2.ts, 3, 12))
->Cls : Symbol(Cls, Decl(mod2.ts, 0, 8))
-
-export const foo = new Cls().foo();
->foo : Symbol(foo, Decl(mod2.ts, 4, 12))
->new Cls().foo : Symbol(Cls.foo, Decl(mod1.ts, 6, 19))
->Cls : Symbol(Cls, Decl(mod2.ts, 0, 8))
->foo : Symbol(Cls.foo, Decl(mod1.ts, 6, 19))
-
-export const bar = Cls.bar();
->bar : Symbol(bar, Decl(mod2.ts, 5, 12))
->Cls.bar : Symbol(Cls.bar, Decl(mod1.ts, 9, 19))
->Cls : Symbol(Cls, Decl(mod2.ts, 0, 8))
->bar : Symbol(Cls.bar, Decl(mod1.ts, 9, 19))
-
-=== /types/lib/index.d.ts ===
-
-
-interface Lib { x }
->Lib : Symbol(Lib, Decl(index.d.ts, 0, 0))
->x : Symbol(Lib.x, Decl(index.d.ts, 2, 15))
-
-=== /main.ts ===
-export class Cls {
->Cls : Symbol(Cls, Decl(main.ts, 0, 0), Decl(mod1.ts, 5, 25), Decl(mod1.ts, 8, 5))
-
-    x
->x : Symbol(Cls.x, Decl(main.ts, 0, 18))
-}
-
-=== /mod1.ts ===
-/// <reference types="lib" />
-
-import {Cls} from "./main";
->Cls : Symbol(Cls, Decl(mod1.ts, 2, 8))
-
-Cls.prototype.foo = function() { return undefined; }
->Cls.prototype.foo : Symbol(Cls.foo, Decl(mod1.ts, 6, 19))
->Cls.prototype : Symbol(Cls.prototype)
->Cls : Symbol(Cls, Decl(mod1.ts, 2, 8))
->prototype : Symbol(Cls.prototype)
->foo : Symbol(Cls.foo, Decl(mod1.ts, 6, 19))
->undefined : Symbol(undefined)
-
-declare module "./main" {
-    interface Cls {
->Cls : Symbol(Cls, Decl(main.ts, 0, 0), Decl(mod1.ts, 5, 25), Decl(mod1.ts, 8, 5))
-
-        foo(): Lib;
->foo : Symbol(Cls.foo, Decl(mod1.ts, 6, 19))
->Lib : Symbol(Lib, Decl(index.d.ts, 0, 0))
-    }
-    namespace Cls {
->Cls : Symbol(Cls, Decl(main.ts, 0, 0), Decl(mod1.ts, 5, 25), Decl(mod1.ts, 8, 5))
-
-        function bar(): Lib;
->bar : Symbol(bar, Decl(mod1.ts, 9, 19))
->Lib : Symbol(Lib, Decl(index.d.ts, 0, 0))
-    }
-}
-
diff --git a/tests/baselines/reference/typeReferenceDirectives12.types b/tests/baselines/reference/typeReferenceDirectives12.types
deleted file mode 100644
index bd429b91f1c..00000000000
--- a/tests/baselines/reference/typeReferenceDirectives12.types
+++ /dev/null
@@ -1,73 +0,0 @@
-=== /mod2.ts ===
-import { Cls } from "./main";
->Cls : typeof Cls
-
-import "./mod1";
-
-export const cls = Cls;
->cls : typeof Cls
->Cls : typeof Cls
-
-export const foo = new Cls().foo();
->foo : Lib
->new Cls().foo() : Lib
->new Cls().foo : () => Lib
->new Cls() : Cls
->Cls : typeof Cls
->foo : () => Lib
-
-export const bar = Cls.bar();
->bar : Lib
->Cls.bar() : Lib
->Cls.bar : () => Lib
->Cls : typeof Cls
->bar : () => Lib
-
-=== /types/lib/index.d.ts ===
-
-
-interface Lib { x }
->Lib : Lib
->x : any
-
-=== /main.ts ===
-export class Cls {
->Cls : Cls
-
-    x
->x : any
-}
-
-=== /mod1.ts ===
-/// <reference types="lib" />
-
-import {Cls} from "./main";
->Cls : typeof Cls
-
-Cls.prototype.foo = function() { return undefined; }
->Cls.prototype.foo = function() { return undefined; } : () => any
->Cls.prototype.foo : () => Lib
->Cls.prototype : Cls
->Cls : typeof Cls
->prototype : Cls
->foo : () => Lib
->function() { return undefined; } : () => any
->undefined : undefined
-
-declare module "./main" {
-    interface Cls {
->Cls : Cls
-
-        foo(): Lib;
->foo : () => Lib
->Lib : Lib
-    }
-    namespace Cls {
->Cls : typeof Cls
-
-        function bar(): Lib;
->bar : () => Lib
->Lib : Lib
-    }
-}
-
diff --git a/tests/cases/compiler/outModuleConcatUnspecifiedModuleKind.ts b/tests/cases/compiler/outModuleConcatUnspecifiedModuleKind.ts
new file mode 100644
index 00000000000..976a47e2f78
--- /dev/null
+++ b/tests/cases/compiler/outModuleConcatUnspecifiedModuleKind.ts
@@ -0,0 +1,8 @@
+// @target: ES5
+// @outFile: out.js
+
+// @Filename: a.ts
+export class A { } // module
+
+// @Filename: b.ts
+var x = 0; // global
\ No newline at end of file