diff --git a/doc/source/whatsnew/v0.23.1.txt b/doc/source/whatsnew/v0.23.1.txt
index 35d150dc263b8..9c19d4d6bbaad 100644
--- a/doc/source/whatsnew/v0.23.1.txt
+++ b/doc/source/whatsnew/v0.23.1.txt
@@ -48,6 +48,11 @@ Groupby/Resample/Rolling
 ^^^^^^^^^^^^^^^^^^^^^^^^
 
 - Bug in :func:`DataFrame.agg` where applying multiple aggregation functions to a :class:`DataFrame` with duplicated column names would cause a stack overflow (:issue:`21063`)
+
+Strings
+^^^^^^^
+
+- Bug in :meth:`Series.str.replace()` where the method throws `TypeError` on Python 3.5.2 (:issue: `21078`)
 -
 
 Conversion
diff --git a/pandas/compat/__init__.py b/pandas/compat/__init__.py
index 12517372fedd1..5ae22694d0da7 100644
--- a/pandas/compat/__init__.py
+++ b/pandas/compat/__init__.py
@@ -425,7 +425,7 @@ def raise_with_traceback(exc, traceback=Ellipsis):
 
 # In Python 3.7, the private re._pattern_type is removed.
 # Python 3.5+ have typing.re.Pattern
-if PY35:
+if PY36:
     import typing
     re_type = typing.re.Pattern
 else:
diff --git a/pandas/tests/test_compat.py b/pandas/tests/test_compat.py
index ead9ba1e26e2d..79d3aad493182 100644
--- a/pandas/tests/test_compat.py
+++ b/pandas/tests/test_compat.py
@@ -4,9 +4,10 @@
 """
 
 import pytest
+import re
 from pandas.compat import (range, zip, map, filter, lrange, lzip, lmap,
                            lfilter, builtins, iterkeys, itervalues, iteritems,
-                           next, get_range_parameters, PY2)
+                           next, get_range_parameters, PY2, re_type)
 
 
 class TestBuiltinIterators(object):
@@ -89,3 +90,7 @@ def test_get_range_parameters(self, start, stop, step):
         assert start_result == start_expected
         assert stop_result == stop_expected
         assert step_result == step_expected
+
+
+def test_re_type():
+    assert isinstance(re.compile(''), re_type)