diff --git a/Lib/email/mime/text.py b/Lib/email/mime/text.py
index 5a5e39b..80ff950 100644
--- a/Lib/email/mime/text.py
+++ b/Lib/email/mime/text.py
@@ -14,7 +14,7 @@
 class MIMEText(MIMENonMultipart):
     """Class for generating text/* type MIME documents."""
 
-    def __init__(self, _text, _subtype='plain', _charset='us-ascii'):
+    def __init__(self, _text, _subtype='plain', _charset=None):
         """Create a text/* type MIME document.
 
         _text is the string for this message object.
@@ -25,16 +25,18 @@
         header.  This defaults to "us-ascii".  Note that as a side-effect, the
         Content-Transfer-Encoding header will also be set.
         """
-        MIMENonMultipart.__init__(self, 'text', _subtype,
-                                  **{'charset': _charset})
 
-        # If _charset was defualted, check to see see if there are non-ascii
-        # characters present. Default to utf-8 if there are.
+        # If no _charset was specified, check to see see if there are non-ascii
+        # characters present. If not, use 'us-ascii', otherwise use utf-8.
         # XXX: This can be removed once #7304 is fixed.
-        if _charset =='us-ascii':
+        if _charset is None:
             try:
-                _text.encode(_charset)
+                _text.encode('us-ascii')
+                _charset = 'us-ascii'
             except UnicodeEncodeError:
                 _charset = 'utf-8'
 
+        MIMENonMultipart.__init__(self, 'text', _subtype,
+                                  **{'charset': _charset})
+
         self.set_payload(_text, _charset)