diff --git a/README.md b/README.md
index 04bd475de..0c7d0ea06 100644
--- a/README.md
+++ b/README.md
@@ -48,7 +48,9 @@ engine.render(tpl, {name: 'alice'})
 var engine = Liquid({
     root: path.resolve(__dirname, 'views/'),  // for layouts and includes
     extname: '.liquid',
-    cache: false
+    cache: false,
+    strict_filters: false,       // default: false
+    strict_variables: false      // default: false
 });
 engine.renderFile("hello.liquid", {name: 'alice'})
     .then(function(html){
@@ -69,26 +71,11 @@ Defaults to `process.cwd()`
 
 * `cache` indicates whether or not to cache resolved templates. Defaults to `false`.
 
-## Strict Rendering
+* `strict_filters` is used to enable strict filter existence. If set to `false`, undefined filters will be rendered as empty string. Otherwise, undefined filters will cause an exception. Defaults to `false`.
 
-Undefined filters and variables will be rendered as empty string by default.
-Enable strict rendering to throw errors upon undefined variables/filters:
-
-```javascript
-var opts = {
-    strict_variables: true, 
-    strict_filters: true
-};
-engine.parseAndRender("{{ foo }}", {}, opts).catch(function(err){
-    // err.message === undefined variable: foo
-});
-engine.parseAndRender("{{ 'foo' | filter1 }}", {}, opts).catch(function(err){
-    // err.message === undefined filter: filter1
-});
-// Note: the below opts also work:
-// engine.render(tpl, ctx, opts)
-// engine.renderFile(path, ctx, opts)
-```
+* `strict_variables` is used to enable strict variable derivation. 
+If set to `false`, undefined variables will be rendered as empty string.
+Otherwise, undefined variables will cause an exception. Defaults to `false`.
 
 ## Use with Express.js
 
@@ -104,7 +91,8 @@ app.set('view engine', 'liquid');       // set to default
 
 > There's an Express demo [here](demo/express/).
 
-Note: includes and layouts lookup path should always be specified by `Liquid({root: []})`.
+When using with Express.js, partials(includes and layouts) will be looked up in
+both Liquid `root` and Express `views` directories.
 
 ## Use in Browser
 
diff --git a/index.js b/index.js
index f6dd36bd1..b979807a9 100644
--- a/index.js
+++ b/index.js
@@ -38,6 +38,7 @@ var _engine = {
         return this.parser.parse(tokens);
     },
     render: function(tpl, ctx, opts) {
+        opts = _.assign({}, this.options, opts);
         var scope = Scope.factory(ctx, opts);
         return this.renderer.renderTemplates(tpl, scope);
     },
@@ -73,6 +74,7 @@ var _engine = {
     },
     lookup: function(filepath, root) {
         root = this.options.root.concat(root || []);
+        root = _.uniq(root);
         var paths = root.map(root => path.resolve(root, filepath));
         return anySeries(paths, path => statFileAsync(path).then(() => path))
             .catch((e) => {
diff --git a/src/filter.js b/src/filter.js
index d75848cb0..0c03e5660 100644
--- a/src/filter.js
+++ b/src/filter.js
@@ -21,7 +21,7 @@ module.exports = function() {
             if (typeof filter !== 'function'){
                 return {
                     name: name,
-                    error: new Error(`undefined filter: ${name}`)
+                    error: new TypeError(`undefined filter: ${name}`)
                 };
             }
 
diff --git a/src/scope.js b/src/scope.js
index 72e808ce0..960f1029f 100644
--- a/src/scope.js
+++ b/src/scope.js
@@ -4,9 +4,9 @@ const assert = require('./util/assert.js');
 const referenceError = /undefined variable|Cannot read property .* of undefined/;
 
 var Scope = {
-    getAll: function(str) {
+    getAll: function() {
         var ctx = {};
-        for (i = this.scopes.length - 1; i >= 0; i--) {
+        for (var i = this.scopes.length - 1; i >= 0; i--) {
             _.assign(ctx, this.scopes[i]);
         }
         return ctx;
@@ -26,13 +26,6 @@ var Scope = {
             throw new TypeError('undefined variable: ' + str);
         }
     },
-    safeGet: function(str) {
-        try {
-            var val = this.safeGet(str);
-        } catch (e) {;
-        }
-        return val;
-    },
     set: function(k, v) {
         this.setPropertyByPath(this.scopes[this.scopes.length - 1], k, v);
         return this;
@@ -112,7 +105,7 @@ var Scope = {
                 }
                 // foo["bar"]
                 else {
-                    var j = str.indexOf(delemiter, i + 2);
+                    j = str.indexOf(delemiter, i + 2);
                     assert(j !== -1, `unbalanced ${delemiter}: ${str}`);
                     name = str.slice(i + 2, j);
                     seq.push(name);
diff --git a/src/util/underscore.js b/src/util/underscore.js
index 1e8b8cb25..67744f421 100644
--- a/src/util/underscore.js
+++ b/src/util/underscore.js
@@ -25,27 +25,75 @@ function forOwn(object, iteratee) {
     return object;
 }
 
-function assign(dst, src) {
-    dst = dst || {};
+/*
+ * Assigns own enumerable string keyed properties of source objects to the destination object. 
+ * Source objects are applied from left to right. 
+ * Subsequent sources overwrite property assignments of previous sources.  
+ *
+ * Note: This method mutates object and is loosely based on Object.assign.
+ *
+ * @param {Object} object The destination object.
+ * @param {...Object} sources The source objects.
+ * @return {Object} Returns object.
+ */
+function assign(object) {
+    object = isObject(object) ? object : {};
+    var srcs = Array.prototype.slice.call(arguments, 1);
+    srcs.forEach(function(src) {
+        _assignBinary(object, src);
+    });
+    return object;
+}
+
+function _assignBinary(dst, src) {
+    if (!dst) return dst;
     forOwn(src, function(v, k) {
         dst[k] = v;
     });
     return dst;
 }
 
+
 function isArray(value) {
     return value instanceof Array;
 }
 
-function echo(prefix){
+function echo(prefix) {
     return v => {
         console.log('[' + prefix + ']', v);
         return v;
     };
 }
 
+function uniq(arr) {
+    var u = {},
+        a = [];
+    for (var i = 0, l = arr.length; i < l; ++i) {
+        if (u.hasOwnProperty(arr[i])) {
+            continue;
+        }
+        a.push(arr[i]);
+        u[arr[i]] = 1;
+    }
+    return a;
+}
+
+/*
+ * Checks if value is the language type of Object. 
+ * (e.g. arrays, functions, objects, regexes, new Number(0), and new String(''))
+ * @param {any} value The value to check.
+ * @return {Boolean} Returns true if value is an object, else false.
+ */
+function isObject(value) {
+    return value !== null && typeof value === 'object';
+}
+
 exports.isString = isString;
 exports.isArray = isArray;
+exports.isObject = isObject;
+
 exports.forOwn = forOwn;
 exports.assign = assign;
+exports.uniq = uniq;
+
 exports.echo = echo;
diff --git a/test/liquid.js b/test/liquid.js
index 593f61186..eec3f9bbf 100644
--- a/test/liquid.js
+++ b/test/liquid.js
@@ -1,5 +1,4 @@
 const chai = require("chai");
-const should = chai.should();
 const expect = chai.expect;
 const Liquid = require('..');
 const mock = require('mock-fs');
@@ -152,10 +151,10 @@ describe('liquid', function() {
         it('should be disabled by default', function() {
             return engine.renderFile('files/foo')
                 .then(x => expect(x).to.equal('foo'))
-                .then(x => mock({
+                .then(() => mock({
                     '/root/files/foo.html': 'bar'
                 }))
-                .then(x => engine.renderFile('files/foo'))
+                .then(() => engine.renderFile('files/foo'))
                 .then(x => expect(x).to.equal('bar'));
         });
         it('should respect cache=true option', function() {
@@ -166,10 +165,10 @@ describe('liquid', function() {
             });
             return engine.renderFile('files/foo')
                 .then(x => expect(x).to.equal('foo'))
-                .then(x => mock({
+                .then(() => mock({
                     '/root/files/foo.html': 'bar'
                 }))
-                .then(x => engine.renderFile('files/foo'))
+                .then(() => engine.renderFile('files/foo'))
                 .then(x => expect(x).to.equal('foo'));
         });
     });
diff --git a/test/util/error.js b/test/util/error.js
index a1f3aa1e2..293b330f2 100644
--- a/test/util/error.js
+++ b/test/util/error.js
@@ -1,65 +1,103 @@
 const chai = require("chai");
 const expect = chai.expect;
 const mock = require('mock-fs');
+chai.use(require("chai-as-promised"));
 
-var engine = require('../..')(), ctx;
-
-function test(promise, cb){
-    return promise
-        .then((result) => {
-            return cb({});
-        })
-        .catch((error) => {
-            return cb(error);
-        });
-}
+var engine = require('../..')();
+var strictEngine = require('../..')({
+    strict_variables: true,
+    strict_filters: true
+});
 
 describe('error', function() {
 
-    it('should throw TokenizationError when tag illegal', function() {
-        return test(engine.parseAndRender('{% . a %}', {}), function(err){
-            expect(err.name).to.equal('TokenizationError');
-            expect(err.message).to.equal('illegal tag: {% . a %}');
-            expect(err.input).to.equal('{% . a %}');
-            expect(err.line).to.equal(1);
+    describe('TokenizationError', function() {
+        it('should throw TokenizationError when tag illegal', function() {
+            return engine.parseAndRender('{% . a %}', {}).catch(function(err) {
+                expect(err.name).to.equal('TokenizationError');
+                expect(err.message).to.equal('illegal tag: {% . a %}');
+                expect(err.input).to.equal('{% . a %}');
+                expect(err.line).to.equal(1);
+            });
+        });
+        it('should throw TokenizationError when tag syntax illegal', function() {
+            return engine.parseAndRender('{% . a }', {}).catch(function(err) {
+                expect(err.name).to.equal('TokenizationError');
+                expect(err.message).to.equal('illegal tag: {% . a }');
+                expect(err.input).to.equal('{% . a }');
+                expect(err.line).to.equal(1);
+            });
+        });
+        it('should throw TokenizationError when filter syntax illegal', function() {
+            return engine.parseAndRender('{{ a|| }}', {}).catch(function(err) {
+                expect(err.name).to.equal('TokenizationError');
+                expect(err.message).to.equal('{{ a|| }');
+                expect(err.input).to.equal('{{ a|| }');
+                expect(err.line).to.equal(1);
+            });
         });
     });
 
-    it('should throw correct error info', function() {
-        return test(engine.parseAndRender('{%if true%}\naaa{%endif%}\n{% -a %}\n3', {}), function(err){
-            expect(err.input).to.equal('{% -a %}');
-            expect(err.line).to.equal(3);
+    describe('TypeError', function() {
+        it('should not throw when variable undefined by default', function() {
+            return expect(engine.parseAndRender('X{{a}}Y')).to.eventually.equal('XY');
+        });
+        it('should throw TypeError when variable not defined', function() {
+            return expect(strictEngine.parseAndRender('{{a}}')).to.eventually
+                .be.rejected
+                .then(function(e){
+                    expect(e).to.have.property('name', 'TypeError');
+                    expect(e).to.have.property('message', 'undefined variable: a');
+                });
+        });
+        it('should throw TypeError when filter not defined', function() {
+            return expect(strictEngine.parseAndRender('{{1 | a}}')).to.eventually
+                .be.rejected
+                .then(function(e){
+                    expect(e).to.have.property('name', 'TypeError');
+                    expect(e).to.have.property('message', 'undefined filter: a');
+                });
         });
     });
 
-    it('should throw correct error info for files', function() {
-        mock({
-            "/foo.html": '<html>\n<head>\n\n{% raw %}\n\n'
+    describe('ParseError', function() {
+        it('should throw correct error info', function() {
+            var src = '{%if true%}\naaa{%endif%}\n{% -a %}\n3';
+            return engine.parseAndRender(src).catch(function(err) {
+                expect(err.name).to.equal('ParseError');
+                expect(err.input).to.equal('{% -a %}');
+                expect(err.line).to.equal(3);
+            });
         });
-        return test(engine.renderFile('/foo.html', {}), function(err){
-            mock.restore();
-            expect(err.input).to.equal('{% raw %}');
-            expect(err.line).to.equal(4);
-            expect(err.file).to.equal('/foo.html');
+        it('should throw correct error info for files', function() {
+            mock({
+                "/foo.html": '<html>\n<head>\n\n{% raw %}\n\n'
+            });
+            return engine.renderFile('/foo.html').catch(function(err) {
+                mock.restore();
+                expect(err.name).to.equal('ParseError');
+                expect(err.input).to.equal('{% raw %}');
+                expect(err.line).to.equal(4);
+                expect(err.file).to.equal('/foo.html');
+            });
+        });
+
+        it('should throw ParseError when tag not exist', function() {
+            return engine.parseAndRender('{% a %}').catch(function(err) {
+                expect(err.name).to.equal('ParseError');
+                expect(err.message).to.equal('tag a not found');
+                expect(err.input).to.equal('{% a %}');
+                expect(err.line).to.equal(1);
+            });
+        });
+
+        it('should throw ParseError when tag not closed', function() {
+            return engine.parseAndRender('{% if %}').catch(function(err) {
+                expect(err.name).to.equal('ParseError');
+                expect(err.message).to.equal('tag {% if %} not closed');
+                expect(err.input).to.equal('{% if %}');
+                expect(err.line).to.equal(1);
+            });
         });
     });
-
-    it('should throw ParseError when tag not exist', function() {
-        return test(engine.parseAndRender('{% a %}', {}), function(err){
-            expect(err.name).to.equal('ParseError');
-            expect(err.message).to.equal('tag a not found');
-            expect(err.input).to.equal('{% a %}');
-            expect(err.line).to.equal(1);
-        });
-    });
-
-    it('should throw ParseError when tag not closed', function() {
-        return test(engine.parseAndRender('{% if %}', {}), function(err){
-            expect(err.name).to.equal('ParseError');
-            expect(err.message).to.equal('tag {% if %} not closed');
-            expect(err.input).to.equal('{% if %}');
-            expect(err.line).to.equal(1);
-        });
-    });
-
 });
diff --git a/test/util/underscore.js b/test/util/underscore.js
index 0cb39ee5f..fad3dd0a1 100644
--- a/test/util/underscore.js
+++ b/test/util/underscore.js
@@ -11,7 +11,7 @@ describe('util/underscore', function() {
             expect(_.isString('foo')).to.be.true;
         });
         it('should return true String instance', function() {
-            expect(_.isString(new String('foo'))).to.be.true;
+            expect(_.isString(String('foo'))).to.be.true;
         });
         it('should return false for 123 ', function() {
             expect(_.isString(123)).to.be.false;
@@ -53,7 +53,7 @@ describe('util/underscore', function() {
             expect(_.isArray("foo")).to.be.false;
         });
     });
-    describe('.echo()', function(){
+    describe('.echo()', function() {
         it('should be transparent', function() {
             expect(_.echo('foo')('bar')).to.equal('bar');
         });
@@ -63,4 +63,49 @@ describe('util/underscore', function() {
             expect(log).to.have.been.calledWith('[foo]', 'bar');
         });
     });
+    describe('.assign()', function() {
+        it('should handle null dst', function() {
+            expect(_.assign(null, {
+                foo: 'bar'
+            })).to.deep.equal({
+                foo: 'bar'
+            });
+        });
+        it('should assign 2 objects', function() {
+            var src = {
+                foo: 'foo',
+                bar: 'bar'
+            };
+            var dst = {
+                foo: 'bar',
+                kaa: 'kaa'
+            };
+            expect(_.assign(dst, src)).to.deep.equal({
+                foo: 'foo',
+                bar: 'bar',
+                kaa: 'kaa'
+            });
+        });
+        it('should assign 3 objects', function() {
+            expect(_.assign({
+                foo: 'foo'
+            }, {
+                bar: 'bar'
+            }, {
+                car: 'car'
+            })).to.deep.equal({
+                foo: 'foo',
+                bar: 'bar',
+                car: 'car'
+            });
+        });
+    });
+    describe('.uniq()', function() {
+        it('should handle empty array', function() {
+            expect(_.uniq([])).to.deep.equal([]);
+        });
+        it('should do uniq', function() {
+            expect(_.uniq([1, 'a', 'a', 1])).to.deep.equal([1, 'a']);
+        });
+    });
 });