diff --git a/src/core.js b/src/core.js
index cae3a90e39..39e68c4fce 100644
--- a/src/core.js
+++ b/src/core.js
@@ -422,8 +422,11 @@ jQuery.extend({
 	},
 
 	isPlainObject: function( obj ) {
+		var key;
 		// Not plain objects:
 		// - Any object or value whose internal [[Class]] property is not "[object Object]"
+		//   ...Unless it was a constructor, whose prototype property was paved over by
+		//   by a plain object at its declaration. #13571
 		// - DOM nodes
 		// - window
 		if ( jQuery.type( obj ) !== "object" || obj.nodeType || jQuery.isWindow( obj ) ) {
@@ -442,9 +445,11 @@ jQuery.extend({
 			return false;
 		}
 
-		// If the function hasn't returned already, we're confident that
-		// |obj| is a plain object, created by {} or constructed with new Object
-		return true;
+		// Own properties are enumerated firstly, so to speed up,
+		// if last one is own, then all properties are own.
+		for ( key in obj ) {}
+
+		return key === undefined || core_hasOwn.call( obj, key );
 	},
 
 	isEmptyObject: function( obj ) {
diff --git a/test/unit/core.js b/test/unit/core.js
index 7c840993b6..7936777111 100644
--- a/test/unit/core.js
+++ b/test/unit/core.js
@@ -296,9 +296,9 @@ test("type", function() {
 });
 
 asyncTest("isPlainObject", function() {
-	expect(15);
+	expect(16);
 
-	var pass, iframe, doc,
+	var pass, iframe, doc, c, extended,
 		fn = function() {};
 
 	// The use case that we want to match
@@ -360,6 +360,45 @@ asyncTest("isPlainObject", function() {
 	} catch(e) {
 		window.iframeDone( Object, "iframes not supported" );
 	}
+
+	// #13571
+	function C() {}
+	C.prototype = {
+		x: 1
+	};
+	c = new C();
+
+	extended = jQuery.extend( true, {}, {
+		target: c
+	});
+
+	strictEqual(
+		extended.target, c,
+		"Instances, whose constructor defined its prototype by assigning a plain object, " +
+		"will lie about their true identity to preserve a broken user-code expectation"
+	);
+	//
+	// The test above is broken and tests a broken feature, to support a misinformed
+	// assumption, as documented here:
+	//
+	// http://bugs.jquery.com/ticket/13571#comment:4
+	//
+	// It will not pass if the object being assigned as the prototype
+	// has no properties:
+	//
+	// function C() {}
+	// C.prototype = {};
+	// c = new C();
+
+	// extended = jQuery.extend( true, {}, {
+	// 	target: c
+	// });
+
+	// strictEqual( extended.target, c, "Undetectable, will fail every time" );
+	//
+	// The solution is to reset the constructor property of your plain object prototypes.
+	//
+	//
 });
 
 test("isFunction", function() {