diff --git a/Parsedown.php b/Parsedown.php
index 9bdae22..87c10a6 100755
--- a/Parsedown.php
+++ b/Parsedown.php
@@ -1212,7 +1212,7 @@ class Parsedown
         {
             if (preg_match('/^\s*\[(.*?)\]/', $remainder, $matches))
             {
-                $definition = $matches[1] ? $matches[1] : $Element['text'];
+                $definition = strlen($matches[1]) ? $matches[1] : $Element['text'];
                 $definition = strtolower($definition);
 
                 $extent += strlen($matches[0]);