diff --git a/Lib/StringIO.py b/Lib/StringIO.py
index cc88e9d..9225c05 100644
--- a/Lib/StringIO.py
+++ b/Lib/StringIO.py
@@ -39,7 +39,7 @@
 class StringIO:
     def __init__(self, buf = ''):
         # Force self.buf to be a string or unicode
-        if type(buf) is not types.UnicodeType:
+        if not isinstance(buf, types.UnicodeType):
             buf = str(buf)
         self.buf = buf
         self.len = len(buf)
@@ -138,7 +138,7 @@
             raise ValueError, "I/O operation on closed file"
         if not s: return
         # Force s to be a string or unicode
-        if type(s) is not types.UnicodeType:
+        if not isinstance(s, types.UnicodeType):
             s = str(s)
         if self.pos > self.len:
             self.buflist.append('\0'*(self.pos - self.len))