diff --git a/lib/RuntimeTemplate.js b/lib/RuntimeTemplate.js
index 08b9e636d65..feb9b3e505c 100644
--- a/lib/RuntimeTemplate.js
+++ b/lib/RuntimeTemplate.js
@@ -200,7 +200,7 @@ class RuntimeTemplate {
 
 		// when the first two args are expression, we need to prepend "" + to force string
 		// concatenation instead of number addition.
-		return typeof args[0] !== "string" && args[1] !== "string"
+		return typeof args[0] !== "string" && typeof args[1] !== "string"
 			? `"" + ${str}`
 			: str;
 	}
diff --git a/test/RuntimeTemplate.unittest.js b/test/RuntimeTemplate.unittest.js
new file mode 100644
index 00000000000..1cd083e3036
--- /dev/null
+++ b/test/RuntimeTemplate.unittest.js
@@ -0,0 +1,78 @@
+"use strict";
+
+const RuntimeTemplate = require("../lib/RuntimeTemplate");
+const RequestShortener = require("../lib/RequestShortener");
+
+describe("RuntimeTemplate.concatenation", () => {
+	it("no args", () => {
+		const runtimeTemplate = new RuntimeTemplate(
+			undefined,
+			{ environment: { templateLiteral: false } },
+			new RequestShortener(__dirname)
+		);
+		expect(runtimeTemplate.concatenation()).toBe('""');
+	});
+
+	it("1 arg", () => {
+		const runtimeTemplate = new RuntimeTemplate(
+			undefined,
+			{ environment: { templateLiteral: false } },
+			new RequestShortener(__dirname)
+		);
+		expect(runtimeTemplate.concatenation({ expr: 1 })).toBe('"" + 1');
+		expect(runtimeTemplate.concatenation("str")).toBe('"str"');
+	});
+
+	it("es5", () => {
+		const runtimeTemplate = new RuntimeTemplate(
+			undefined,
+			{ environment: { templateLiteral: false } },
+			new RequestShortener(__dirname)
+		);
+
+		expect(
+			runtimeTemplate.concatenation({ expr: "__webpack__.p" }, "str/a")
+		).toBe('__webpack__.p + "str/a"');
+		expect(
+			runtimeTemplate.concatenation(
+				{ expr: "__webpack__.p" },
+				{ expr: "str.a" },
+				"str"
+			)
+		).toBe('"" + __webpack__.p + str.a + "str"');
+		expect(runtimeTemplate.concatenation("a", "b", { expr: 1 })).toBe(
+			'"a" + "b" + 1'
+		);
+		expect(runtimeTemplate.concatenation("a", { expr: 1 }, "b")).toBe(
+			'"a" + 1 + "b"'
+		);
+	});
+
+	describe("es6", () => {
+		const runtimeTemplate = new RuntimeTemplate(
+			undefined,
+			{ environment: { templateLiteral: true } },
+			new RequestShortener(__dirname)
+		);
+
+		it("should prefer shorten variant #1", () => {
+			expect(runtimeTemplate.concatenation({ expr: 1 }, "a", { expr: 2 })).toBe(
+				'1 + "a" + 2'
+			);
+		});
+
+		it("should prefer shorten variant #2", () => {
+			expect(
+				runtimeTemplate.concatenation({ expr: 1 }, "a", { expr: 2 }, "b")
+			).toBe('1 + "a" + 2 + "b"');
+		});
+
+		it("should prefer shorten variant #3", () => {
+			/* eslint-disable no-template-curly-in-string */
+			expect(runtimeTemplate.concatenation("a", { expr: 1 }, "b")).toBe(
+				"`a${1}b`"
+			);
+			/* eslint-enable */
+		});
+	});
+});