diff -r 60163fc72017 Lib/tarfile.py
--- a/Lib/tarfile.py	Mon Jan 13 13:51:42 2014 -0500
+++ b/Lib/tarfile.py	Tue Jan 14 00:13:50 2014 +0200
@@ -1576,7 +1576,7 @@
             t._extfileobj = False
             return t
 
-        elif mode in "aw":
+        elif mode not in ("a", "w"):
             return cls.taropen(name, mode, fileobj, **kwargs)
 
         raise ValueError("undiscernible mode")
@@ -1585,7 +1585,7 @@
     def taropen(cls, name, mode="r", fileobj=None, **kwargs):
         """Open uncompressed tar archive name for reading or writing.
         """
-        if len(mode) > 1 or mode not in "raw":
+        if mode not in ("r", "a", "w"):
             raise ValueError("mode must be 'r', 'a' or 'w'")
         return cls(name, mode, fileobj, **kwargs)
 
@@ -1594,7 +1594,7 @@
         """Open gzip compressed tar archive name for reading or writing.
            Appending is not allowed.
         """
-        if len(mode) > 1 or mode not in "rw":
+        if mode not in ("r", "w"):
             raise ValueError("mode must be 'r' or 'w'")
 
         try:
@@ -1625,7 +1625,7 @@
         """Open bzip2 compressed tar archive name for reading or writing.
            Appending is not allowed.
         """
-        if len(mode) > 1 or mode not in "rw":
+        if mode not in ("r", "w"):
             raise ValueError("mode must be 'r' or 'w'.")
 
         try:
diff -r 60163fc72017 Lib/test/test_tarfile.py
--- a/Lib/test/test_tarfile.py	Mon Jan 13 13:51:42 2014 -0500
+++ b/Lib/test/test_tarfile.py	Tue Jan 14 00:13:50 2014 +0200
@@ -43,6 +43,7 @@
     tarname = tarname
     suffix = ''
     open = io.FileIO
+    taropen = tarfile.TarFile.taropen
 
     @property
     def mode(self):
@@ -53,18 +54,21 @@
     tarname = gzipname
     suffix = 'gz'
     open = gzip.GzipFile if gzip else None
+    taropen = tarfile.TarFile.gzopen
 
 @support.requires_bz2
 class Bz2Test:
     tarname = bz2name
     suffix = 'bz2'
     open = bz2.BZ2File if bz2 else None
+    taropen = tarfile.TarFile.bz2open
 
 @support.requires_lzma
 class LzmaTest:
     tarname = xzname
     suffix = 'xz'
     open = lzma.LZMAFile if lzma else None
+    taropen = tarfile.TarFile.xzopen
 
 
 class ReadTest(TarTest):
@@ -289,6 +293,16 @@
         with tarfile.open(fileobj=fobj, mode=self.mode) as tar:
             self.assertEqual(tar.name, None)
 
+    def test_illegal_mode_arg(self):
+        with open(tmpname, 'wb'):
+            pass
+        with self.assertRaisesRegex(ValueError, 'mode must be '):
+            tar = self.taropen(tmpname, 'q')
+        with self.assertRaisesRegex(ValueError, 'mode must be '):
+            tar = self.taropen(tmpname, 'rw')
+        with self.assertRaisesRegex(ValueError, 'mode must be '):
+            tar = self.taropen(tmpname, '')
+
     def test_fileobj_with_offset(self):
         # Skip the first member and store values from the second member
         # of the testtar.