diff --git a/lib/ExternalModule.js b/lib/ExternalModule.js
index f306710e4fb..8cb5e071c0b 100644
--- a/lib/ExternalModule.js
+++ b/lib/ExternalModule.js
@@ -74,7 +74,9 @@ class ExternalModule extends Module {
 			.slice(1)
 			.map(r => `[${JSON.stringify(r)}]`)
 			.join("");
-		return `module.exports = require(${moduleName})${objectLookup};`;
+		return `module.exports = require(${JSON.stringify(
+			moduleName
+		)})${objectLookup};`;
 	}
 
 	checkExternalVariable(variableToCheck, request) {
@@ -94,15 +96,25 @@ class ExternalModule extends Module {
 	}
 
 	getSourceForDefaultCase(optional, request) {
+		if (!Array.isArray(request)) {
+			// make it an array as the look up works the same basically
+			request = [request];
+		}
+
+		const variableName = request[0];
 		const missingModuleError = optional
-			? this.checkExternalVariable(request, request)
+			? this.checkExternalVariable(variableName, request.join("."))
 			: "";
-		return `${missingModuleError}module.exports = ${request};`;
+		const objectLookup = request
+			.slice(1)
+			.map(r => `[${JSON.stringify(r)}]`)
+			.join("");
+		return `${missingModuleError}module.exports = ${variableName}${objectLookup};`;
 	}
 
 	getSourceString(runtime) {
 		const request =
-			typeof this.request === "object"
+			typeof this.request === "object" && !Array.isArray(this.request)
 				? this.request[this.externalType]
 				: this.request;
 		switch (this.externalType) {
diff --git a/schemas/WebpackOptions.json b/schemas/WebpackOptions.json
index 01513196134..ac11bfcdd3d 100644
--- a/schemas/WebpackOptions.json
+++ b/schemas/WebpackOptions.json
@@ -123,6 +123,9 @@
               {
                 "type": "object"
               },
+              {
+                "$ref": "#/definitions/common.arrayOfStringValues"
+              },
               {
                 "type": "boolean"
               }
diff --git a/test/ExternalModule.unittest.js b/test/ExternalModule.unittest.js
deleted file mode 100644
index 075c4600919..00000000000
--- a/test/ExternalModule.unittest.js
+++ /dev/null
@@ -1,308 +0,0 @@
-/* globals describe, it, beforeEach */
-"use strict";
-
-const ExternalModule = require("../lib/ExternalModule");
-const OriginalSource = require("webpack-sources").OriginalSource;
-const RawSource = require("webpack-sources").RawSource;
-
-describe("ExternalModule", () => {
-	let externalModule;
-	let request;
-	let type;
-	beforeEach(() => {
-		request = "some/request";
-		type = "some-type";
-		externalModule = new ExternalModule(request, type, `${type} ${request}`);
-	});
-	describe("#identifier", () => {
-		it("returns an identifier for this module", () => {
-			const expected = `external "${request}"`;
-			expect(externalModule.identifier()).toBe(expected);
-		});
-	});
-
-	describe("#readableIdentifier", () => {
-		it("returns an identifier for this module", () => {
-			const expected = `external "${request}"`;
-			expect(externalModule.identifier()).toBe(expected);
-		});
-	});
-
-	describe("#needRebuild", () => {
-		it("always returns false", () => {
-			expect(externalModule.needRebuild()).toBe(false);
-		});
-	});
-
-	describe("#size", () => {
-		it("always returns 42", () => {
-			expect(externalModule.size()).toBe(42);
-		});
-	});
-
-	describe("#source", () => {
-		it("calls getSource with the result of getSourceString", () => {
-			// set up
-			const expectedString = "something expected stringy";
-			const expectedSource = "something expected source";
-			externalModule.getSource = jest.fn(() => expectedSource);
-			externalModule.getSourceString = jest.fn(() => expectedString);
-
-			// invoke
-			const result = externalModule.source();
-
-			// check
-			expect(externalModule.getSource.mock.calls.length).toBe(1);
-			expect(externalModule.getSourceString.mock.calls.length).toBe(1);
-			expect(externalModule.getSource.mock.calls[0][0]).toBe(expectedString);
-			expect(result).toEqual(expectedSource);
-		});
-	});
-
-	describe("#getSource", () => {
-		describe("given it should use source maps", () => {
-			beforeEach(() => {
-				externalModule.useSourceMap = true;
-			});
-			it("returns an instance of OriginalSource", () => {
-				// set up
-				const someSourceString = "some source string";
-
-				// invoke
-				const result = externalModule.getSource(someSourceString);
-
-				// check
-				expect(result).toBeInstanceOf(OriginalSource);
-			});
-		});
-		describe("given it does not use source maps", () => {
-			beforeEach(() => {
-				externalModule.useSourceMap = false;
-			});
-			it("returns an instance of RawSource", () => {
-				// set up
-				const someSourceString = "some source string";
-
-				// invoke
-				const result = externalModule.getSource(someSourceString);
-
-				// check
-				expect(result).toBeInstanceOf(RawSource);
-			});
-		});
-	});
-
-	describe("#getSourceForGlobalVariableExternal", () => {
-		describe("given an array as variable name in the global namespace", () => {
-			it("use the array as lookup in the global object", () => {
-				// set up
-				const type = "window";
-				const varName = ["foo", "bar"];
-				const expected =
-					'(function() { module.exports = window["foo"]["bar"]; }());';
-
-				// invoke
-				const result = externalModule.getSourceForGlobalVariableExternal(
-					varName,
-					type
-				);
-
-				// check
-				expect(result).toEqual(expected);
-			});
-		});
-		describe("given an single variable name", () => {
-			it("look it up in the global namespace", () => {
-				// set up
-				const type = "window";
-				const varName = "foo";
-				const expected = '(function() { module.exports = window["foo"]; }());';
-
-				// invoke
-				const result = externalModule.getSourceForGlobalVariableExternal(
-					varName,
-					type
-				);
-
-				// check
-				expect(result).toEqual(expected);
-			});
-		});
-	});
-
-	describe("#getSourceForCommonJsExternal", () => {
-		describe("given an array as names in the global namespace", () => {
-			it("use the first to require a module and the rest as lookup on the required module", () => {
-				// set up
-				const varName = ["module", "look", "up"];
-				const expected = 'module.exports = require(module)["look"]["up"];';
-
-				// invoke
-				const result = externalModule.getSourceForCommonJsExternal(
-					varName,
-					type
-				);
-
-				// check
-				expect(result).toEqual(expected);
-			});
-		});
-		describe("given an single variable name", () => {
-			it("require a module with that name", () => {
-				// set up
-				const type = "window";
-				const varName = "foo";
-				const expected = 'module.exports = require("foo");';
-
-				// invoke
-				const result = externalModule.getSourceForCommonJsExternal(
-					varName,
-					type
-				);
-
-				// check
-				expect(result).toEqual(expected);
-			});
-		});
-	});
-
-	describe("#checkExternalVariable", () => {
-		it("creates a check that fails if a variable does not exist", () => {
-			// set up
-			const variableToCheck = "foo";
-			const request = "bar";
-			const expected = `if(typeof foo === 'undefined') {var e = new Error("Cannot find module 'bar'"); e.code = 'MODULE_NOT_FOUND'; throw e;}
-`;
-
-			// invoke
-			const result = externalModule.checkExternalVariable(
-				variableToCheck,
-				request
-			);
-
-			// check
-			expect(result).toEqual(expected);
-		});
-	});
-
-	describe("#getSourceForAmdOrUmdExternal", () => {
-		it("looks up a global variable as specified by the id", () => {
-			// set up
-			const id = "someId";
-			const optional = false;
-			const expected = "module.exports = __WEBPACK_EXTERNAL_MODULE_someId__;";
-
-			// invoke
-			const result = externalModule.getSourceForAmdOrUmdExternal(
-				id,
-				optional,
-				request
-			);
-
-			// check
-			expect(result).toEqual(expected);
-		});
-		describe("given an optional check is set", function() {
-			it("ads a check for the existence of the variable before looking it up", () => {
-				// set up
-				const id = "someId";
-				const optional = true;
-				const expected = `if(typeof __WEBPACK_EXTERNAL_MODULE_someId__ === 'undefined') {var e = new Error("Cannot find module 'some/request'"); e.code = 'MODULE_NOT_FOUND'; throw e;}
-module.exports = __WEBPACK_EXTERNAL_MODULE_someId__;`;
-
-				// invoke
-				const result = externalModule.getSourceForAmdOrUmdExternal(
-					id,
-					optional,
-					request
-				);
-
-				// check
-				expect(result).toEqual(expected);
-			});
-		});
-	});
-
-	describe("#getSourceForDefaultCase", () => {
-		it("returns the given request as lookup", () => {
-			// set up
-			const optional = false;
-			const expected = "module.exports = some/request;";
-
-			// invoke
-			const result = externalModule.getSourceForDefaultCase(optional, request);
-
-			// check
-			expect(result).toEqual(expected);
-		});
-		describe("given an optional check is requested", function() {
-			it("checks for the existence of the request setting it", () => {
-				// set up
-				const optional = true;
-				const expected = `if(typeof some/request === 'undefined') {var e = new Error("Cannot find module 'some/request'"); e.code = 'MODULE_NOT_FOUND'; throw e;}
-module.exports = some/request;`;
-
-				// invoke
-				const result = externalModule.getSourceForDefaultCase(
-					optional,
-					request
-				);
-
-				// check
-				expect(result).toEqual(expected);
-			});
-		});
-	});
-
-	describe("#updateHash", () => {
-		let hashedText;
-		let hash;
-		beforeEach(() => {
-			hashedText = "";
-			hash = {
-				update: text => {
-					hashedText += text;
-				}
-			};
-			externalModule.id = 12345678;
-			externalModule.updateHash(hash);
-		});
-		it("updates hash with request", () => {
-			expect(hashedText).toMatch("some/request");
-		});
-		it("updates hash with type", () => {
-			expect(hashedText).toMatch("some-type");
-		});
-		it("updates hash with module id", () => {
-			expect(hashedText).toMatch("12345678");
-		});
-	});
-
-	describe("#updateHash without optional", () => {
-		let hashedText;
-		let hash;
-		beforeEach(() => {
-			hashedText = "";
-			hash = {
-				update: text => {
-					hashedText += text;
-				}
-			};
-			// Note no set of `externalModule.optional`, which crashed externals in 3.7.0
-			externalModule.id = 12345678;
-			externalModule.updateHash(hash);
-		});
-		it("updates hash with request", () => {
-			expect(hashedText).toMatch("some/request");
-		});
-		it("updates hash with type", () => {
-			expect(hashedText).toMatch("some-type");
-		});
-		it("updates hash with optional flag", () => {
-			expect(hashedText).toMatch("false");
-		});
-		it("updates hash with module id", () => {
-			expect(hashedText).toMatch("12345678");
-		});
-	});
-});
diff --git a/test/configCases/externals/externals-array/index.js b/test/configCases/externals/externals-array/index.js
new file mode 100644
index 00000000000..a7dedba652b
--- /dev/null
+++ b/test/configCases/externals/externals-array/index.js
@@ -0,0 +1,4 @@
+it("should not fail on optional externals", function() {
+	const external = require("external");
+	expect(external).toBe(EXPECTED);
+});
diff --git a/test/configCases/externals/externals-array/webpack.config.js b/test/configCases/externals/externals-array/webpack.config.js
new file mode 100644
index 00000000000..af6b62d059c
--- /dev/null
+++ b/test/configCases/externals/externals-array/webpack.config.js
@@ -0,0 +1,26 @@
+const webpack = require("../../../../");
+module.exports = [
+	{
+		output: {
+			libraryTarget: "commonjs2"
+		},
+		externals: {
+			external: ["webpack", "version"]
+		},
+		plugins: [
+			new webpack.DefinePlugin({
+				EXPECTED: JSON.stringify(webpack.version)
+			})
+		]
+	},
+	{
+		externals: {
+			external: ["Array", "isArray"]
+		},
+		plugins: [
+			new webpack.DefinePlugin({
+				EXPECTED: "Array.isArray"
+			})
+		]
+	}
+];