diff --git a/src/calibre/gui2/css_transform_rules.py b/src/calibre/gui2/css_transform_rules.py
index fefb04ba43..9b041df563 100644
--- a/src/calibre/gui2/css_transform_rules.py
+++ b/src/calibre/gui2/css_transform_rules.py
@@ -233,7 +233,10 @@ class Tester(Dialog):  # {{{
     def do_test(self):
         decl = safe_parser().parseString(self.value)
         transform_sheet(self.rules, decl)
-        self.result.load_text('/* %s */\n\n%s' % (_('Resulting stylesheet'), decl.cssText), 'css')
+        css = decl.cssText
+        if isinstance(css, bytes):
+            css = css.decode('utf-8')
+        self.result.load_text('/* %s */\n\n%s' % (_('Resulting stylesheet'), css), 'css')
 
     def sizeHint(self):
         return QSize(800, 600)