diff --git a/src/calibre/gui2/tweak_book/diff/main.py b/src/calibre/gui2/tweak_book/diff/main.py
index 9e8022868f..e747184780 100644
--- a/src/calibre/gui2/tweak_book/diff/main.py
+++ b/src/calibre/gui2/tweak_book/diff/main.py
@@ -108,9 +108,11 @@ def get_decoded_raw(name):
         if syntax in {'html', 'xml'}:
             raw = xml_to_unicode(raw, verbose=True)[0]
         else:
-            m = re.search(r"coding[:=]\s*([-\w.]+)", raw[:1024], flags=re.I)
+            m = re.search(br"coding[:=]\s*([-\w.]+)", raw[:1024], flags=re.I)
             if m is not None and m.group(1) != '8bit':
                 enc = m.group(1)
+                if enc == b'unicode':
+                    enc = 'utf-8'
             else:
                 enc = force_encoding(raw, verbose=True)
             try: