diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 2b059e29d09b5..cda19a0783540 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -132,6 +132,8 @@ namespace ts {
         const evolvingArrayTypes: EvolvingArrayType[] = [];
 
         const unknownSymbol = createSymbol(SymbolFlags.Property, "unknown");
+        const untypedModuleSymbol = createSymbol(SymbolFlags.ValueModule, "<untyped>");
+        untypedModuleSymbol.exports = createMap<Symbol>();
         const resolvingSymbol = createSymbol(0, "__resolving__");
 
         const anyType = createIntrinsicType(TypeFlags.Any, "any");
@@ -1158,7 +1160,7 @@ namespace ts {
 
             if (moduleSymbol) {
                 let exportDefaultSymbol: Symbol;
-                if (isShorthandAmbientModuleSymbol(moduleSymbol)) {
+                if (isUntypedOrShorthandAmbientModuleSymbol(moduleSymbol)) {
                     exportDefaultSymbol = moduleSymbol;
                 }
                 else {
@@ -1238,7 +1240,7 @@ namespace ts {
             if (targetSymbol) {
                 const name = specifier.propertyName || specifier.name;
                 if (name.text) {
-                    if (isShorthandAmbientModuleSymbol(moduleSymbol)) {
+                    if (isUntypedOrShorthandAmbientModuleSymbol(moduleSymbol)) {
                         return moduleSymbol;
                     }
 
@@ -1491,15 +1493,19 @@ namespace ts {
                 if (isForAugmentation) {
                     const diag = Diagnostics.Invalid_module_name_in_augmentation_Module_0_resolves_to_an_untyped_module_at_1_which_cannot_be_augmented;
                     error(errorNode, diag, moduleReference, resolvedModule.resolvedFileName);
+                    return undefined;
                 }
                 else if (compilerOptions.noImplicitAny && moduleNotFoundError) {
                     error(errorNode,
                         Diagnostics.Could_not_find_a_declaration_file_for_module_0_1_implicitly_has_an_any_type,
                         moduleReference,
                         resolvedModule.resolvedFileName);
+                    return undefined;
                 }
-                // Failed imports and untyped modules are both treated in an untyped manner; only difference is whether we give a diagnostic first.
-                return undefined;
+                // Unlike a failed import, an untyped module produces a dummy symbol.
+                // This is checked for by `isUntypedOrShorthandAmbientModuleSymbol`.
+                // This must be different than `unknownSymbol` because `getBaseConstructorTypeOfClass` won't fail for `unknownSymbol`.
+                return untypedModuleSymbol;
             }
 
             if (moduleNotFoundError) {
@@ -3668,7 +3674,7 @@ namespace ts {
         function getTypeOfFuncClassEnumModule(symbol: Symbol): Type {
             const links = getSymbolLinks(symbol);
             if (!links.type) {
-                if (symbol.flags & SymbolFlags.Module && isShorthandAmbientModuleSymbol(symbol)) {
+                if (symbol.flags & SymbolFlags.Module && isUntypedOrShorthandAmbientModuleSymbol(symbol)) {
                     links.type = anyType;
                 }
                 else {
@@ -20508,7 +20514,7 @@ namespace ts {
 
         function moduleExportsSomeValue(moduleReferenceExpression: Expression): boolean {
             let moduleSymbol = resolveExternalModuleName(moduleReferenceExpression.parent, moduleReferenceExpression);
-            if (!moduleSymbol || isShorthandAmbientModuleSymbol(moduleSymbol)) {
+            if (!moduleSymbol || isUntypedOrShorthandAmbientModuleSymbol(moduleSymbol)) {
                 // If the module is not found or is shorthand, assume that it may export a value.
                 return true;
             }
diff --git a/src/compiler/utilities.ts b/src/compiler/utilities.ts
index 39d53a9a9c053..f4c505b945031 100644
--- a/src/compiler/utilities.ts
+++ b/src/compiler/utilities.ts
@@ -425,8 +425,8 @@ namespace ts {
     }
 
     /** Given a symbol for a module, checks that it is either an untyped import or a shorthand ambient module. */
-    export function isShorthandAmbientModuleSymbol(moduleSymbol: Symbol): boolean {
-        return isShorthandAmbientModule(moduleSymbol.valueDeclaration);
+    export function isUntypedOrShorthandAmbientModuleSymbol(moduleSymbol: Symbol): boolean {
+        return !moduleSymbol.declarations || isShorthandAmbientModule(moduleSymbol.valueDeclaration);
     }
 
     function isShorthandAmbientModule(node: Node): boolean {
diff --git a/src/services/findAllReferences.ts b/src/services/findAllReferences.ts
index 2d33ca7b651d5..76e1e9b23d7f8 100644
--- a/src/services/findAllReferences.ts
+++ b/src/services/findAllReferences.ts
@@ -133,7 +133,7 @@ namespace ts.FindAllReferences {
                 return { symbol };
             }
 
-            if (ts.isShorthandAmbientModuleSymbol(aliasedSymbol)) {
+            if (ts.isUntypedOrShorthandAmbientModuleSymbol(aliasedSymbol)) {
                 return { symbol, shorthandModuleSymbol: aliasedSymbol };
             }
 
diff --git a/tests/baselines/reference/extendsUntypedModule.errors.txt b/tests/baselines/reference/extendsUntypedModule.errors.txt
new file mode 100644
index 0000000000000..9a1c423535261
--- /dev/null
+++ b/tests/baselines/reference/extendsUntypedModule.errors.txt
@@ -0,0 +1,14 @@
+/a.ts(2,17): error TS2507: Type 'any' is not a constructor function type.
+
+
+==== /a.ts (1 errors) ====
+    import Foo from "foo";
+    class A extends Foo { }
+                    ~~~
+!!! error TS2507: Type 'any' is not a constructor function type.
+    
+==== /node_modules/foo/index.js (0 errors) ====
+    // Test that extending an untyped module is an error, unlike extending unknownSymbol.
+    
+    This file is not read.
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/extendsUntypedModule.js b/tests/baselines/reference/extendsUntypedModule.js
new file mode 100644
index 0000000000000..f86ded7e6cb9f
--- /dev/null
+++ b/tests/baselines/reference/extendsUntypedModule.js
@@ -0,0 +1,33 @@
+//// [tests/cases/compiler/extendsUntypedModule.ts] ////
+
+//// [index.js]
+// Test that extending an untyped module is an error, unlike extending unknownSymbol.
+
+This file is not read.
+
+//// [a.ts]
+import Foo from "foo";
+class A extends Foo { }
+
+
+//// [a.js]
+"use strict";
+var __extends = (this && this.__extends) || (function () {
+    var extendStatics = Object.setPrototypeOf ||
+        ({ __proto__: [] } instanceof Array && function (d, b) { d.__proto__ = b; }) ||
+        function (d, b) { for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p]; };
+    return function (d, b) {
+        extendStatics(d, b);
+        function __() { this.constructor = d; }
+        d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
+    };
+})();
+exports.__esModule = true;
+var foo_1 = require("foo");
+var A = (function (_super) {
+    __extends(A, _super);
+    function A() {
+        return _super !== null && _super.apply(this, arguments) || this;
+    }
+    return A;
+}(foo_1["default"]));
diff --git a/tests/cases/compiler/extendsUntypedModule.ts b/tests/cases/compiler/extendsUntypedModule.ts
new file mode 100644
index 0000000000000..8eaf6b3833a83
--- /dev/null
+++ b/tests/cases/compiler/extendsUntypedModule.ts
@@ -0,0 +1,9 @@
+// Test that extending an untyped module is an error, unlike extending unknownSymbol.
+// @noImplicitReferences: true
+
+// @Filename: /node_modules/foo/index.js
+This file is not read.
+
+// @Filename: /a.ts
+import Foo from "foo";
+class A extends Foo { }
diff --git a/tests/cases/fourslash/untypedModuleImport.ts b/tests/cases/fourslash/untypedModuleImport.ts
index 1854010d5b68f..433c584e3cfcd 100644
--- a/tests/cases/fourslash/untypedModuleImport.ts
+++ b/tests/cases/fourslash/untypedModuleImport.ts
@@ -12,7 +12,7 @@ verify.numberOfErrorsInCurrentFile(0);
 
 goTo.marker("fooModule");
 verify.goToDefinitionIs([]);
-verify.quickInfoIs("");
+verify.quickInfoIs("module <untyped>");
 verify.noReferences();
 
 goTo.marker("foo");