diff --git a/package.json b/package.json
index 77012599b46..dc977a1c572 100644
--- a/package.json
+++ b/package.json
@@ -44,7 +44,7 @@
   },
   "homepage": "https://github.com/rollup/rollup",
   "devDependencies": {
-    "acorn": "^4.0.1",
+    "acorn": "4.0.4",
     "buble": "^0.15.1",
     "chalk": "^1.1.3",
     "codecov.io": "^0.1.6",
diff --git a/src/ast/nodes/ConditionalExpression.js b/src/ast/nodes/ConditionalExpression.js
index 9c7b4445b87..1e3180dffc0 100644
--- a/src/ast/nodes/ConditionalExpression.js
+++ b/src/ast/nodes/ConditionalExpression.js
@@ -54,10 +54,18 @@ export default class ConditionalExpression extends Node {
 			else if ( this.testValue ) {
 				code.remove( this.start, this.consequent.start );
 				code.remove( this.consequent.end, this.end );
+				if ( this.consequent.type === 'SequenceExpression' ) {
+					code.insertRight( this.consequent.start, '(' );
+					code.insertLeft( this.consequent.end, ')' );
+				}
 				this.consequent.render( code, es );
 			} else {
 				code.remove( this.start, this.alternate.start );
 				code.remove( this.alternate.end, this.end );
+				if ( this.alternate.type === 'SequenceExpression' ) {
+					code.insertRight( this.alternate.start, '(' );
+					code.insertLeft( this.alternate.end, ')' );
+				}
 				this.alternate.render( code, es );
 			}
 		}
diff --git a/test/form/conditional-put-parens-around-sequence/_config.js b/test/form/conditional-put-parens-around-sequence/_config.js
new file mode 100644
index 00000000000..a040c92d6de
--- /dev/null
+++ b/test/form/conditional-put-parens-around-sequence/_config.js
@@ -0,0 +1,3 @@
+module.exports = {
+	description: 'put parens around sequences if conditional simplified (#1311)'
+};
diff --git a/test/form/conditional-put-parens-around-sequence/_expected/amd.js b/test/form/conditional-put-parens-around-sequence/_expected/amd.js
new file mode 100644
index 00000000000..046859685f6
--- /dev/null
+++ b/test/form/conditional-put-parens-around-sequence/_expected/amd.js
@@ -0,0 +1,10 @@
+define(function () { 'use strict';
+
+	var a = (1, 2, 3 );
+	var b = (4, 5, 6 );
+	foo( a, b );
+
+	// verify works with no whitespace
+	bar((1,2),(7,8));
+
+});
diff --git a/test/form/conditional-put-parens-around-sequence/_expected/cjs.js b/test/form/conditional-put-parens-around-sequence/_expected/cjs.js
new file mode 100644
index 00000000000..9bedf35cd81
--- /dev/null
+++ b/test/form/conditional-put-parens-around-sequence/_expected/cjs.js
@@ -0,0 +1,8 @@
+'use strict';
+
+var a = (1, 2, 3 );
+var b = (4, 5, 6 );
+foo( a, b );
+
+// verify works with no whitespace
+bar((1,2),(7,8));
diff --git a/test/form/conditional-put-parens-around-sequence/_expected/es.js b/test/form/conditional-put-parens-around-sequence/_expected/es.js
new file mode 100644
index 00000000000..f33e372379e
--- /dev/null
+++ b/test/form/conditional-put-parens-around-sequence/_expected/es.js
@@ -0,0 +1,6 @@
+var a = (1, 2, 3 );
+var b = (4, 5, 6 );
+foo( a, b );
+
+// verify works with no whitespace
+bar((1,2),(7,8));
diff --git a/test/form/conditional-put-parens-around-sequence/_expected/iife.js b/test/form/conditional-put-parens-around-sequence/_expected/iife.js
new file mode 100644
index 00000000000..97612fbc8da
--- /dev/null
+++ b/test/form/conditional-put-parens-around-sequence/_expected/iife.js
@@ -0,0 +1,11 @@
+(function () {
+	'use strict';
+
+	var a = (1, 2, 3 );
+	var b = (4, 5, 6 );
+	foo( a, b );
+
+	// verify works with no whitespace
+	bar((1,2),(7,8));
+
+}());
diff --git a/test/form/conditional-put-parens-around-sequence/_expected/umd.js b/test/form/conditional-put-parens-around-sequence/_expected/umd.js
new file mode 100644
index 00000000000..2885e8ce5ee
--- /dev/null
+++ b/test/form/conditional-put-parens-around-sequence/_expected/umd.js
@@ -0,0 +1,14 @@
+(function (global, factory) {
+	typeof exports === 'object' && typeof module !== 'undefined' ? factory() :
+	typeof define === 'function' && define.amd ? define(factory) :
+	(factory());
+}(this, (function () { 'use strict';
+
+	var a = (1, 2, 3 );
+	var b = (4, 5, 6 );
+	foo( a, b );
+
+	// verify works with no whitespace
+	bar((1,2),(7,8));
+
+})));
diff --git a/test/form/conditional-put-parens-around-sequence/main.js b/test/form/conditional-put-parens-around-sequence/main.js
new file mode 100644
index 00000000000..60c7062e66b
--- /dev/null
+++ b/test/form/conditional-put-parens-around-sequence/main.js
@@ -0,0 +1,6 @@
+var a = true  ? ( 1, 2, 3 ) : ( 4, 5, 6 );
+var b = false ? ( 1, 2, 3 ) : ( 4, 5, 6 );
+foo( a, b );
+
+// verify works with no whitespace
+bar(true?(1,2):(3,4),false?(5,6):(7,8));