diff --git a/src/vs/platform/contextkey/common/contextkey.ts b/src/vs/platform/contextkey/common/contextkey.ts
index 2cae5106d8a3ca527ed304f3ab7aa9db2815ea19..6f0fb98af61bfdb7cef122c7b2a98b150ff02a14 100644
--- a/src/vs/platform/contextkey/common/contextkey.ts
+++ b/src/vs/platform/contextkey/common/contextkey.ts
@@ -17,6 +17,8 @@ STATIC_VALUES.set('isWindows', isWindows);
 STATIC_VALUES.set('isWeb', isWeb);
 STATIC_VALUES.set('isMacNative', isMacintosh && !isWeb);
 
+const hasOwnProperty = Object.prototype.hasOwnProperty;
+
 export const enum ContextKeyExprType {
 	False = 0,
 	True = 1,
@@ -138,7 +140,7 @@ export abstract class ContextKeyExpr {
 
 		if (serializedOne.indexOf(' in ') >= 0) {
 			let pieces = serializedOne.split(' in ');
-			return ContextKeyInExpr.create(pieces[0].trim(), this._deserializeValue(pieces[1], strict));
+			return ContextKeyInExpr.create(pieces[0].trim(), pieces[1].trim());
 		}
 
 		if (/^\!\s*/.test(serializedOne)) {
@@ -451,8 +453,8 @@ export class ContextKeyInExpr implements IContextKeyExpression {
 			return (source.indexOf(item) >= 0);
 		}
 
-		if (typeof item === 'string' && typeof source === 'object' && source !== undefined && source !== null) {
-			return item in source;
+		if (typeof item === 'string' && typeof source === 'object' && source !== null) {
+			return hasOwnProperty.call(source, item);
 		}
 		return false;
 	}
@@ -462,7 +464,7 @@ export class ContextKeyInExpr implements IContextKeyExpression {
 	}
 
 	public keys(): string[] {
-		return [this.key];
+		return [this.key, this.valueKey];
 	}
 
 	public map(mapFnc: IContextKeyExprMapper): ContextKeyInExpr {
diff --git a/src/vs/platform/contextkey/test/common/contextkey.test.ts b/src/vs/platform/contextkey/test/common/contextkey.test.ts
index c7784c888e18b282e092bc2ff14e4d7befc1bef3..042012f1d3355e5638ad09b5e931833e842b4d68 100644
--- a/src/vs/platform/contextkey/test/common/contextkey.test.ts
+++ b/src/vs/platform/contextkey/test/common/contextkey.test.ts
@@ -150,4 +150,19 @@ suite('ContextKeyExpr', () => {
 		t('a || b', 'c && d', 'a && c && d || b && c && d');
 		t('a || b', 'c && d || e', 'a && e || b && e || a && c && d || b && c && d');
 	});
+
+	test('ContextKeyInExpr', () => {
+		const ainb = ContextKeyExpr.deserialize('a in b')!;
+		assert.equal(ainb.evaluate(createContext({ 'a': 3, 'b': [3, 2, 1] })), true);
+		assert.equal(ainb.evaluate(createContext({ 'a': 3, 'b': [1, 2, 3] })), true);
+		assert.equal(ainb.evaluate(createContext({ 'a': 3, 'b': [1, 2] })), false);
+		assert.equal(ainb.evaluate(createContext({ 'a': 3 })), false);
+		assert.equal(ainb.evaluate(createContext({ 'a': 3, 'b': null })), false);
+		assert.equal(ainb.evaluate(createContext({ 'a': 'x', 'b': ['x'] })), true);
+		assert.equal(ainb.evaluate(createContext({ 'a': 'x', 'b': ['y'] })), false);
+		assert.equal(ainb.evaluate(createContext({ 'a': 'x', 'b': {} })), false);
+		assert.equal(ainb.evaluate(createContext({ 'a': 'x', 'b': { 'x': false } })), true);
+		assert.equal(ainb.evaluate(createContext({ 'a': 'x', 'b': { 'x': true } })), true);
+		assert.equal(ainb.evaluate(createContext({ 'a': 'prototype', 'b': {} })), false);
+	});
 });