diff --git a/src/calibre/gui2/viewer/convert_book.py b/src/calibre/gui2/viewer/convert_book.py
index 13b04582f8..996a105469 100644
--- a/src/calibre/gui2/viewer/convert_book.py
+++ b/src/calibre/gui2/viewer/convert_book.py
@@ -314,8 +314,14 @@ def find_tests():
                 self.ae(instance['key'], key)
                 open(os.path.join(temp_path, instance['path'], 'sentinel'), 'wb').write(b'test')
 
+            def set_data(x):
+                if not isinstance(x, bytes):
+                    x = x.encode('utf-8')
+                with open(book_src, 'wb') as f:
+                    f.write(x)
+
             book_src = os.path.join(self.tdir, 'book.epub')
-            open(book_src, 'wb').write(b'a')
+            set_data('a')
             path = prepare_book(book_src, convert_func=convert_mock)
             self.ae(open(os.path.join(path, 'sentinel'), 'rb').read(), b'test')
 
@@ -324,7 +330,7 @@ def find_tests():
             self.ae(path, second_path)
 
             # Test that changing the book updates the cache
-            open(book_src, 'wb').write(b'bc')
+            set_data('bc')
             third_path = prepare_book(book_src, convert_func=convert_mock)
             self.assertNotEqual(path, third_path)
 
@@ -335,21 +341,21 @@ def find_tests():
             self.assertNotEqual(third_path, fourth_path)
 
             # Test cache expiry
-            open(book_src, 'wb').write(b'bcd')
+            set_data('bcd')
             prepare_book(book_src, convert_func=convert_mock, max_age=-1000)
             self.ae([], os.listdir(os.path.join(book_cache_dir(), 'f')))
 
             # Test modifying a book and opening it repeatedly leaves only
             # a single entry for it in the cache
-            prepare_book(book_src, convert_func=convert_mock, force_expire=True)
+            opath = prepare_book(book_src, convert_func=convert_mock, force_expire=True)
             finished_entries = os.listdir(os.path.join(book_cache_dir(), 'f'))
             self.ae(len(finished_entries), 1)
-            open(book_src, 'wb').write(b'bcde')
-            prepare_book(book_src, convert_func=convert_mock, force_expire=True)
+            set_data('bcde' * 4096)
+            npath = prepare_book(book_src, convert_func=convert_mock, force_expire=True)
             new_finished_entries = os.listdir(os.path.join(book_cache_dir(), 'f'))
             self.ae(len(new_finished_entries), 1)
-            self.assertNotEqual(finished_entries, new_finished_entries)
-            open(book_src, 'wb').write(b'bcdef')
+            self.assertNotEqual(opath, npath)
+            set_data('bcdef')
             prepare_book(book_src, convert_func=convert_mock, max_age=-1000, force_expire=True)
             self.ae([], os.listdir(os.path.join(book_cache_dir(), 'f')))
             with cache_lock() as f:
@@ -358,11 +364,11 @@ def find_tests():
 
             # Test updating cached book
             book_src = os.path.join(self.tdir, 'book2.epub')
-            open(book_src, 'wb').write(b'bb')
+            set_data('bb')
             path = prepare_book(book_src, convert_func=convert_mock)
             self.ae(open(os.path.join(path, 'sentinel'), 'rb').read(), b'test')
             bs = os.stat(book_src)
-            open(book_src, 'wb').write(b'cde')
+            set_data('cde')
             update_book(book_src, bs, name_data_map={'sentinel': b'updated'})
             self.ae(open(os.path.join(path, 'sentinel'), 'rb').read(), b'updated')
             self.ae(1, len(os.listdir(os.path.join(book_cache_dir(), 'f'))))