diff --git a/src/calibre/db/tests/writing.py b/src/calibre/db/tests/writing.py
index 526e541e3c..0dccfc4da6 100644
--- a/src/calibre/db/tests/writing.py
+++ b/src/calibre/db/tests/writing.py
@@ -444,7 +444,7 @@ class WritingTest(BaseTest):
         all_ids = cache.all_book_ids()
         self.assertFalse(cache.has_conversion_options(all_ids))
         self.assertIsNone(cache.conversion_options(1))
-        op1, op2 = {'xx':'yy'}, {'yy':'zz'}
+        op1, op2 = b"{'xx':'yy'}", b"{'yy':'zz'}"
         cache.set_conversion_options({1:op1, 2:op2})
         self.assertTrue(cache.has_conversion_options(all_ids))
         self.assertEqual(cache.conversion_options(1), op1)
diff --git a/src/calibre/utils/__init__.py b/src/calibre/utils/__init__.py
index cef9aefa5e..070191dd1d 100644
--- a/src/calibre/utils/__init__.py
+++ b/src/calibre/utils/__init__.py
@@ -47,6 +47,6 @@ def unpickle_binary_string(data):
 def pickle_binary_string(data):
     # Maintains compatibility with python's pickle module protocol version 2
     import struct
-    from pickle import PROTO, BINSTRING
+    from pickle import PROTO, BINSTRING, STOP
     data = bytes(data)
-    return PROTO + b'\x1b' + BINSTRING + struct.pack(b'<i', len(data)) + data
+    return PROTO + b'\x02' + BINSTRING + struct.pack(b'<i', len(data)) + data + STOP