diff --git a/src/calibre/ebooks/lrf/pylrs/pylrf.py b/src/calibre/ebooks/lrf/pylrs/pylrf.py
index a29c0139d0..d39418399f 100644
--- a/src/calibre/ebooks/lrf/pylrs/pylrf.py
+++ b/src/calibre/ebooks/lrf/pylrs/pylrf.py
@@ -652,7 +652,7 @@ class LrfWriter(object):
         return self.sourceEncoding
 
     def toUnicode(self, string):
-        if isinstance(string, str):
+        if isinstance(string, bytes):
             string = string.decode(self.sourceEncoding)
 
         return string
diff --git a/src/calibre/ebooks/txt/processor.py b/src/calibre/ebooks/txt/processor.py
index d4de4166aa..1dfb2397d5 100644
--- a/src/calibre/ebooks/txt/processor.py
+++ b/src/calibre/ebooks/txt/processor.py
@@ -121,7 +121,7 @@ def create_markdown_object(extensions):
             if hasattr(module, 'makeExtension'):
                 return module.makeExtension(**configs)
             for name, x in vars(module).items():
-                if isinstance(x, type) and issubclass(x, Extension) and x is not Extension:
+                if type(x) is type and issubclass(x, Extension) and x is not Extension:
                     return x(**configs)
             raise ImportError('No extension class in {}'.format(ext_name))
 
diff --git a/src/calibre/gui2/tweak_book/diff/main.py b/src/calibre/gui2/tweak_book/diff/main.py
index 0b41d6beb3..86a7fe099d 100644
--- a/src/calibre/gui2/tweak_book/diff/main.py
+++ b/src/calibre/gui2/tweak_book/diff/main.py
@@ -139,7 +139,7 @@ def string_diff(left, right, left_syntax=None, right_syntax=None, left_name='lef
 
 def file_diff(left, right):
     (raw1, syntax1), (raw2, syntax2) = map(get_decoded_raw, (left, right))
-    if not isinstance(raw1, type(raw2)):
+    if type(raw1) is not type(raw2):
         raw1, raw2 = open(left, 'rb').read(), open(right, 'rb').read()
     cache = Cache()
     cache.set_left(left, raw1), cache.set_right(right, raw2)