diff --git a/composer.json b/composer.json
index 05a2aad..15842e5 100644
--- a/composer.json
+++ b/composer.json
@@ -14,6 +14,6 @@
     }
   },
   "require": {
-    "twig/twig": "^1.24"
+    "opdavies/twig-extensions": "dev-master"
   }
 }
diff --git a/composer.lock b/composer.lock
index 96024c6..3aae140 100644
--- a/composer.lock
+++ b/composer.lock
@@ -4,9 +4,46 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#composer-lock-the-lock-file",
         "This file is @generated automatically"
     ],
-    "hash": "ee4a05d220006a064b4fda49803c239b",
-    "content-hash": "e796f90e007457e383570e7ae0e15248",
+    "hash": "d179d6e4dbbcaeaed647cc67ab4cea30",
+    "content-hash": "cb46a91710b83e4adc6b589f826e282e",
     "packages": [
+        {
+            "name": "opdavies/twig-extensions",
+            "version": "dev-master",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/opdavies/twig-extensions.git",
+                "reference": "6ded44f2cfb250a6049cbbccf9709f1a725097f8"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/opdavies/twig-extensions/zipball/6ded44f2cfb250a6049cbbccf9709f1a725097f8",
+                "reference": "6ded44f2cfb250a6049cbbccf9709f1a725097f8",
+                "shasum": ""
+            },
+            "require": {
+                "twig/twig": "^1.24"
+            },
+            "type": "library",
+            "autoload": {
+                "psr-4": {
+                    "Opdavies\\Twig\\Extensions\\": "src"
+                }
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "MIT"
+            ],
+            "authors": [
+                {
+                    "name": "Oliver Davies",
+                    "email": "oliver@oliverdavies.uk",
+                    "homepage": "https://www.oliverdavies.uk"
+                }
+            ],
+            "description": "A collection of personal Twig extensions.",
+            "time": "2016-07-15 01:40:58"
+        },
         {
             "name": "twig/twig",
             "version": "v1.24.1",
@@ -72,7 +109,9 @@
     "packages-dev": [],
     "aliases": [],
     "minimum-stability": "stable",
-    "stability-flags": [],
+    "stability-flags": {
+        "opdavies/twig-extensions": 20
+    },
     "prefer-stable": false,
     "prefer-lowest": false,
     "platform": [],
diff --git a/src/GmailFilterBuilder.php b/src/GmailFilterBuilder.php
index 4717a08..4fbc3e8 100644
--- a/src/GmailFilterBuilder.php
+++ b/src/GmailFilterBuilder.php
@@ -1,5 +1,7 @@
 <?php
 
+use Opdavies\Twig\Extensions\TwigBooleanStringExtension;
+
 class GmailFilterBuilder
 {
     /**
@@ -30,6 +32,8 @@ class GmailFilterBuilder
             new Twig_Loader_Filesystem(__DIR__.'/../templates')
         );
 
+        $this->twig->addExtension(new TwigBooleanStringExtension());
+
         $this->filters = $filters;
 
         return $this->generate();
diff --git a/templates/filters.xml.twig b/templates/filters.xml.twig
index 9d60998..1314cf7 100644
--- a/templates/filters.xml.twig
+++ b/templates/filters.xml.twig
@@ -8,9 +8,9 @@
             <category term='filter'></category>
             <title>Mail Filter</title>
             <content></content>
-            <apps:property name='shouldArchive' value='{{ filter.isArchive ? 'true' : 'false' }}'/>
-            <apps:property name='shouldNeverSpam' value='{{ filter.isNeverSpam ? 'true' : 'false' }}'/>
-            <apps:property name='shouldTrash' value='{{ filter.isTrash ? 'true' : 'false' }}'/>
+            <apps:property name='shouldArchive' value='{{ filter.isArchive|boolean_string }}'/>
+            <apps:property name='shouldNeverSpam' value='{{ filter.isNeverSpam|boolean_string }}'/>
+            <apps:property name='shouldTrash' value='{{ filter.isTrash|boolean_string }}'/>
             {% for condition in filter.conditions -%}
                 <apps:property name='{{ condition[0] }}' value='{{ condition[1] }}'/>
             {%- endfor %}