diff --git a/src/calibre/devices/kindle/apnx.py b/src/calibre/devices/kindle/apnx.py
index d225c91b90..89b56bc03b 100644
--- a/src/calibre/devices/kindle/apnx.py
+++ b/src/calibre/devices/kindle/apnx.py
@@ -17,7 +17,7 @@ from calibre.ebooks.mobi.reader.headers import MetadataHeader
 from calibre.utils.logging import default_log
 from calibre import prints, fsync
 from calibre.constants import DEBUG
-from polyglot.builtins import range
+from polyglot.builtins import range, as_unicode, as_bytes
 
 
 class APNXBuilder(object):
@@ -37,10 +37,10 @@ class APNXBuilder(object):
 
         with lopen(mobi_file_path, 'rb') as mf:
             ident = PdbHeaderReader(mf).identity()
-            if ident != 'BOOKMOBI':
+            if ident != b'BOOKMOBI':
                 # Check that this is really a MOBI file.
                 raise Exception(_('Not a valid MOBI file. Reports identity of %s') % ident)
-            apnx_meta['acr'] = str(PdbHeaderReader(mf).name())
+            apnx_meta['acr'] = as_unicode(PdbHeaderReader(mf).name(), errors='replace')
 
         # We'll need the PDB name, the MOBI version, and some metadata to make FW 3.4 happy with KF8 files...
         with lopen(mobi_file_path, 'rb') as mf:
@@ -92,7 +92,7 @@ class APNXBuilder(object):
             fsync(apnxf)
 
     def generate_apnx(self, pages, apnx_meta):
-        apnx = ''
+        apnx = b''
 
         if DEBUG:
             prints('APNX META: guid:', apnx_meta['guid'])
@@ -117,12 +117,12 @@ class APNXBuilder(object):
         apnx += struct.pack('>I', 65537)
         apnx += struct.pack('>I', 12 + len(content_header))
         apnx += struct.pack('>I', len(content_header))
-        apnx += content_header
+        apnx += as_bytes(content_header)
         apnx += struct.pack('>H', 1)
         apnx += struct.pack('>H', len(page_header))
         apnx += struct.pack('>H', len(pages))
         apnx += struct.pack('>H', 32)
-        apnx += page_header
+        apnx += as_bytes(page_header)
 
         # Write page values to APNX.
         for page in pages:
diff --git a/src/calibre/devices/usbms/driver.py b/src/calibre/devices/usbms/driver.py
index 806d8bd3a1..5b204904ee 100644
--- a/src/calibre/devices/usbms/driver.py
+++ b/src/calibre/devices/usbms/driver.py
@@ -354,6 +354,7 @@ class USBMS(CLI, Device):
         debug_print('USBMS: adding metadata for %d books'%(len(metadata)))
 
         metadata = iter(metadata)
+        locations = tuple(locations)
         for i, location in enumerate(locations):
             self.report_progress((i+1) / float(len(locations)), _('Adding books to device metadata listing...'))
             info = next(metadata)
diff --git a/src/calibre/ebooks/pdb/header.py b/src/calibre/ebooks/pdb/header.py
index 6e954843e1..86ae4d3bcc 100644
--- a/src/calibre/ebooks/pdb/header.py
+++ b/src/calibre/ebooks/pdb/header.py
@@ -32,7 +32,7 @@ class PdbHeaderReader(object):
 
     def name(self):
         self.stream.seek(0)
-        return re.sub('[^-A-Za-z0-9 ]+', '_', self.stream.read(32).replace('\x00', ''))
+        return re.sub(b'[^-A-Za-z0-9 ]+', b'_', self.stream.read(32).replace(b'\x00', b''))
 
     def full_section_info(self, number):
         if number not in range(0, self.num_sections):