diff --git a/src/calibre/ebooks/pdb/ereader/writer.py b/src/calibre/ebooks/pdb/ereader/writer.py
index cae9f3838c..2f13147180 100644
--- a/src/calibre/ebooks/pdb/ereader/writer.py
+++ b/src/calibre/ebooks/pdb/ereader/writer.py
@@ -67,7 +67,7 @@ class Writer(FormatWriter):
 
         lengths = [len(i) if i not in images else len(i[0]) + len(i[1]) for i in sections]
 
-        pdbHeaderBuilder = PdbHeaderBuilder(IDENTITY, metadata[0].partition('\x00')[0])
+        pdbHeaderBuilder = PdbHeaderBuilder(IDENTITY, metadata[0].partition(b'\x00')[0])
         pdbHeaderBuilder.build_header(lengths, out_stream)
 
         for item in sections:
diff --git a/src/calibre/ebooks/pdb/header.py b/src/calibre/ebooks/pdb/header.py
index 4f3bd475d0..b85be834a1 100644
--- a/src/calibre/ebooks/pdb/header.py
+++ b/src/calibre/ebooks/pdb/header.py
@@ -70,7 +70,9 @@ class PdbHeaderBuilder:
 
     def __init__(self, identity, title):
         self.identity = identity.ljust(3, '\x00')[:8].encode('utf-8')
-        self.title = b'%s\x00' % re.sub('[^-A-Za-z0-9 ]+', '_', title).ljust(31, '\x00')[:31].encode('ascii', 'replace')
+        if isinstance(title, str):
+            title = title.encode('ascii', 'replace')
+        self.title = b'%s\x00' % re.sub(b'[^-A-Za-z0-9 ]+', b'_', title).ljust(31, b'\x00')[:31]
 
     def build_header(self, section_lengths, out_stream):
         '''