make sure using .set throws a warning with the property name

define-helpers/define-helpers.js

@@ -4,6 +4,7 @@ var queues = require("can-queues");
 
 
 var defineHelpers = {
+	// returns `true` if the value was defined and set
 	defineExpando: function(map, prop, value) {
 		// first check if it's already a constructor define
 		var constructorDefines = map._define.definitions;
@@ -13,6 +14,9 @@ var defineHelpers = {
 		// next if it's already on this instances
 		var instanceDefines = map._instanceDefinitions;
 		if(!instanceDefines) {
+			if(Object.isSealed(map)) {
+				return;
+			}
 			Object.defineProperty(map, "_instanceDefinitions", {
 				configurable: true,
 				enumerable: false,

map/map-test.js

@@ -1125,3 +1125,15 @@ QUnit.test("async getters derived from other properties should have correct keyD
 	vm.on('derived', function(){});
 	QUnit.ok(vm._computed.derived.compute.observation.newDependencies.keyDependencies.get(vm).has('source'), 'getter should depend on vm.source');
 });
+
+QUnit.test("setting a property gives a nice error", function(){
+	"use strict;"
+	var VM = DefineMap.extend({});
+	var vm = new VM();
+
+	try {
+		vm.set("fooxyz","bar");
+	} catch(e) {
+		QUnit.ok( e.message.indexOf("fooxyz") >= 0, "talks about the right property being set");
+	}
+});

map/map.js

@@ -1,3 +1,4 @@
+"use strict";
 var Construct = require("can-construct");
 var define = require("can-define");
 var defineHelpers = require("../define-helpers/define-helpers");