diff --git a/Lib/idlelib/PyShell.py b/Lib/idlelib/PyShell.py
index 14f062e09de..3e23e6c0ad2 100644
--- a/Lib/idlelib/PyShell.py
+++ b/Lib/idlelib/PyShell.py
@@ -1265,7 +1265,7 @@ class PseudoFile(object):
         self.encoding = encoding
 
     def write(self, s):
-        if not isinstance(s, str):
+        if not isinstance(s, basestring):
             raise TypeError('must be str, not ' + type(s).__name__)
         self.shell.write(s, self.tags)
 
diff --git a/Lib/idlelib/run.py b/Lib/idlelib/run.py
index 82db38a781c..9389a2b0aad 100644
--- a/Lib/idlelib/run.py
+++ b/Lib/idlelib/run.py
@@ -263,7 +263,7 @@ class _RPCFile(io.TextIOBase):
         return setattr(self.rpc, name, value)
 
     def write(self, s):
-        if not isinstance(s, str):
+        if not isinstance(s, basestring):
             raise TypeError('must be str, not ' + type(s).__name__)
         return self.rpc.write(s)