diff --git a/tests/test_parser.py b/tests/test_parser.py
index 63dd3447..93071453 100644
--- a/tests/test_parser.py
+++ b/tests/test_parser.py
@@ -195,5 +195,8 @@ class TestParser(unittest.TestCase):
         self.encoding_test(html, "ascii")
 
     def encoding_test(self, html, expected):
+        # If chardet is installed Beautiful Soup uses it for encoding detection.
+        # Results for html without a valid charset may differ
+        # based on chardet availability.
         soup = htmlsoup.make_soup(html)
         self.assertEqual(soup.original_encoding, expected)
diff --git a/tox.ini b/tox.ini
index e6918fdc..ca3fd921 100644
--- a/tox.ini
+++ b/tox.ini
@@ -3,6 +3,7 @@ envlist = py36, py37, py38, py39
 
 [base]
 deps =
+    chardet
     pyftpdlib
     parameterized
     pdfminer