diff --git a/.travis.yml b/.travis.yml
index 044083721..a5a21f55b 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -12,6 +12,7 @@ matrix:
         - php: 7.0
         - php: 7.1
         - php: 7.2
+        - php: 7.3
         - php: nightly
     fast_finish: true
 
diff --git a/doc/filters/json_encode.rst b/doc/filters/json_encode.rst
index e93199f63..f02a7a89a 100644
--- a/doc/filters/json_encode.rst
+++ b/doc/filters/json_encode.rst
@@ -14,11 +14,10 @@ The ``json_encode`` filter returns the JSON representation of a value:
 Arguments
 ---------
 
-* ``options``: A bitmask of `json_encode options`_ (``{{
+* ``options``: A bitmask of `json_encode options`_: ``{{
   data|json_encode(constant('JSON_PRETTY_PRINT')) }}``.
-  Combine constants using `Twig's bitwise operators`: ``{{
-  data|json_encode(constant('JSON_PRETTY_PRINT') b-or constant('JSON_HEX_QUOT') }}``)
+  Combine constants using :ref:`bitwise operators<template_logic>`:
+  ``{{ data|json_encode(constant('JSON_PRETTY_PRINT') b-or constant('JSON_HEX_QUOT') }}``
 
 .. _`json_encode`: https://secure.php.net/json_encode
 .. _`json_encode options`: https://secure.php.net/manual/en/json.constants.php
-.. _`Twig's bitwise operators`: https://twig.symfony.com/doc/2.x/templates.html#logic
diff --git a/doc/templates.rst b/doc/templates.rst
index 4205cfb93..792f212fe 100644
--- a/doc/templates.rst
+++ b/doc/templates.rst
@@ -651,6 +651,8 @@ but exists for completeness' sake. The following operators are supported:
 * ``**``: Raises the left operand to the power of the right operand. ``{{ 2 **
   3 }}`` would return ``8``.
 
+.. _template_logic:
+
 Logic
 ~~~~~
 
@@ -666,7 +668,7 @@ You can combine multiple expressions with the following operators:
 
 .. note::
 
-    Twig also support bitwise operators (``b-and``, ``b-xor``, and ``b-or``).
+    Twig also supports bitwise operators (``b-and``, ``b-xor``, and ``b-or``).
 
 .. note::
 
diff --git a/lib/Twig/Extension/Core.php b/lib/Twig/Extension/Core.php
index 321b9773f..3d3877875 100644
--- a/lib/Twig/Extension/Core.php
+++ b/lib/Twig/Extension/Core.php
@@ -1310,7 +1310,7 @@ function twig_include(Twig_Environment $env, $context, $template, $variables = a
         }
     }
 
-    $result = null;
+    $result = '';
     try {
         $result = $env->resolveTemplate($template)->render($variables);
     } catch (Twig_Error_Loader $e) {