diff --git a/src/python/m5/util/grammar.py b/src/python/m5/util/grammar.py
index e09c989..9aba746 100644
--- a/src/python/m5/util/grammar.py
+++ b/src/python/m5/util/grammar.py
@@ -25,7 +25,6 @@
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 import os
-from six import string_types
 
 import ply.lex
 import ply.yacc
@@ -94,7 +93,7 @@
             "'%s' object has no attribute '%s'" % (type(self), attr))
 
     def parse_string(self, data, source='<string>', debug=None, tracking=0):
-        if not isinstance(data, string_types):
+        if not isinstance(data, str):
             raise AttributeError(
                 "argument must be a string, was '%s'" % type(f))
 
@@ -113,7 +112,7 @@
         return result
 
     def parse_file(self, f, **kwargs):
-        if isinstance(f, string_types):
+        if isinstance(f, str):
             source = f
             f = open(f, 'r')
         elif isinstance(f, file):