diff --git a/src/calibre/ebooks/mobi/reader/markup.py b/src/calibre/ebooks/mobi/reader/markup.py
index 4eb16daa1c..e18740b70e 100644
--- a/src/calibre/ebooks/mobi/reader/markup.py
+++ b/src/calibre/ebooks/mobi/reader/markup.py
@@ -335,7 +335,7 @@ def expand_mobi8_markup(mobi8_reader, resource_map, log):
     parts = update_internal_links(mobi8_reader, log)
 
     # Remove pointless markup inserted by kindlegen
-    remove_kindlegen_markup(parts, mobi8_reader.aid_anchor_suffix, mobi8_reader.linked_aids)
+    remove_kindlegen_markup(parts, mobi8_reader.aid_anchor_suffix.decode('utf-8'), mobi8_reader.linked_aids)
 
     # Handle substitutions for the flows pieces first as they may
     # be inlined into the xhtml text
diff --git a/src/calibre/ebooks/mobi/reader/mobi8.py b/src/calibre/ebooks/mobi/reader/mobi8.py
index e87c3369bc..68ab3037b9 100644
--- a/src/calibre/ebooks/mobi/reader/mobi8.py
+++ b/src/calibre/ebooks/mobi/reader/mobi8.py
@@ -337,7 +337,7 @@ class Mobi8Reader(object):
             # See https://www.mobileread.com/forums/showthread.php?t=259557
             m = self.aid_re.match(tag)
             if m is not None:
-                self.linked_aids.add(m.group(1))
+                self.linked_aids.add(m.group(1).decode('utf-8'))
                 return m.group(1) + b'-' + self.aid_anchor_suffix
 
         # No tag found, link to start of file