diff --git a/MarkdownRuntime.php b/MarkdownRuntime.php
index 6a96a52..d1211d2 100644
--- a/MarkdownRuntime.php
+++ b/MarkdownRuntime.php
@@ -20,8 +20,12 @@ public function __construct(MarkdownInterface $converter)
         $this->converter = $converter;
     }
 
-    public function convert(string $body): string
+    public function convert(?string $body): string
     {
+        if (empty($body)) {
+            return '';
+        }
+
         // remove indentation
         if ($white = substr($body, 0, strspn($body, " \t\r\n\0\x0B"))) {
             $body = preg_replace("{^$white}m", '', $body);
diff --git a/Tests/FunctionalTest.php b/Tests/FunctionalTest.php
index 368a046..e0e1ea8 100644
--- a/Tests/FunctionalTest.php
+++ b/Tests/FunctionalTest.php
@@ -81,6 +81,7 @@ public function getMarkdownTests()
 EOF
             , "<h1>Hello</h1>\n+<p>Great!</p>"],
             ["{{ include('html')|markdown_to_html }}", "<h1>Hello</h1>\n+<p>Great!</p>"],
+            ['{{ null|markdown_to_html }}', ""]
         ];
     }
 }