Try with cython code adapted from another function

Author: Bruno P. Kinoshita

pandas/_libs/writers.pyx

@@ -127,19 +127,28 @@ def max_len_string_array(pandas_string[:] arr) -> Py_ssize_t:
 
     for i in range(length):
         val = arr[i]
-        if isinstance(val, str):
-            l = PyString_GET_SIZE(val)
-        elif isinstance(val, bytes):
-            l = PyBytes_GET_SIZE(val)
-        elif isinstance(val, unicode):
-            l = PyUnicode_GET_SIZE(val)
+        l = max_len_string(val)
 
         if l > m:
             m = l
 
     return m
 
 
+def max_len_string(val):
+    """ return the maximum size of a string"""
+    cdef:
+        Py_ssize_t l = 0
+
+    if isinstance(val, str):
+        l = PyString_GET_SIZE(val)
+    elif isinstance(val, bytes):
+        l = PyBytes_GET_SIZE(val)
+    elif isinstance(val, unicode):
+        l = PyUnicode_GET_SIZE(val)
+
+    return l
+
 # ------------------------------------------------------------------
 # PyTables Helpers
 

pandas/core/internals/blocks.py

@@ -8,7 +8,7 @@
 
 import numpy as np
 
-from pandas._libs import internals as libinternals, lib, tslib, tslibs
+from pandas._libs import internals as libinternals, lib, tslib, tslibs, writers
 from pandas._libs.tslibs import Timedelta, conversion, is_null_datetimelike
 import pandas.compat as compat
 from pandas.compat import range, zip
@@ -726,8 +726,9 @@ def to_native_types(self, slicer=None, na_rep='nan', quoting=None,
         mask = isna(values)
 
         if not self.is_object and not quoting:
-            if na_rep and isinstance(na_rep, str):
-                values = values.astype("<U{length}".format(length=len(na_rep)))
+            if na_rep is not None and isinstance(na_rep, (str, bytes)):
+                itemsize = writers.max_len_string(na_rep)
+                values = values.astype("<U{size}".format(size=itemsize))
             else:
                 values = values.astype(str)
         else:

pandas/tests/io/formats/test_to_csv.py

@@ -6,6 +6,8 @@
 import numpy as np
 import pytest
 
+from pandas.compat import StringIO
+
 import pandas as pd
 from pandas import DataFrame, compat
 from pandas.util import testing as tm