diff --git a/src/Support/Tokenizer.php b/src/Support/Tokenizer.php
index 8166e27..2cef252 100644
--- a/src/Support/Tokenizer.php
+++ b/src/Support/Tokenizer.php
@@ -7,7 +7,7 @@ class Tokenizer extends AbstractTokenizer implements TokenizerInterface
 
     public function tokenize($text, $stopwords = [])
     {
-        $text  = mb_strtolower($text);
+        $text  = mb_strtolower($text ?? '');
         $split = preg_split($this->getPattern(), $text, -1, PREG_SPLIT_NO_EMPTY);
         return array_diff($split, $stopwords);
     }